Dark Secrets:
If you see your character in here and feel they have been misinterpreted or want to add further to their story, then just reply with the changes you want noted. People can see it in your post then, and I could probably edit these here:
Angel (Una):
A Maximal agent who was originally from Omicron but was off of the colony at the time of X's attack.
Her dad sent her to the Academy, but near the time of her graduation, it was found that her father was a Predacon double agent specifically working to destabilize the Pax Cybertronia, and he was duly arrested. Angel was also a suspect for a short while before her innocence was proven. Whether or not her father's betrayal was the specific reason, she decided to join the Maximal Security Force.
Her father escaped prison at some point, and the Maximal Security Force decided it would be a great idea to send her to take him back in (smart organization this security force

). Her force confrontation resulted in her near death at the hands of her dad, and a "moral" lecture along with the revelation that he was working for Megatron.
Angel was sent a second time after her father, now with a mandate for his termination. Failure would mean her being labeled as a traitor. She hunted him down, and caught him just about to commit the assassination of a Maximal Elder. When her father pointed his gun at her, he was met with weapons fire, and was killed on the scene.
Feeling unbearable guilt over the situation, she retires and goes in seclusion, all the while maintaining a grudge against the Maximal Elders.
After a few years, she gets her own ship, and goes to Earth, but gets caught in a temporal wave and thus sent into the Beast Wars. Her past problems now manifest themselves in the form of Post Traumatic Distress Disorder, and a hate for firearms.
Starshadow (Starshadow):
Trained for assassination, technological understanding, and combat, Starshadow was a deadly force for Cybertron (despite a mixed Predacon and Maximal heritage, her skills were presumably used for the Predacon cause).
Her parents were executed while she, her sister and they were on a protoform X-related mission (the nature of which I have no further knowledge of). Her parents ended up being executed (which almost certainly means the mission failed), and she and her sister were thrown into a cell with X himself (given canonical history, this would have to have been in the time that Depth Charge was deliberating with Maximal Elders after X's capture, unless protoform X was actually developed in a cruel enough facility to be placed into a cell during his development- the same cell where assassins would also be placed. Either way...).
It has not been specified how they escape, but 4 years later, the Cybertronian Police were sent after the sisters to re-acquire them. By this point, everybody else in their family had been killed in one mission or another.
The chase culminated in a space-bound chase, and Starshadow took the brunt of the attack to divert forces from her sister. Against such hopeless odds, her ship was damaged, so she went into a stasis pod and set the ship to self-destruct.
Our historians are still piecing how she precisely arrived in the time frame of the Beast Wars, but it was likely due to a wave not unlike the one Angel encounters.
Starshadow is conflicted because of her Maximal heritage, but for the most part fights for the Predacon cause in the Beast Wars. Starshadow the player intends for her to change to the Maximal side during "Optimal Situation."
Saber (Night Hunter):
Saber was a Maximal general on Cybertron, but was plagued by something that would have endangered her very life to admit. In an attempt to eradicate the problem she sought the aid of a Predacon scientist (Sonar), but was forced to pledge her loyalty to him in order to get his help. At this point she retired from her position as a general and joined the
Axalon mission as a standing member.
Saber's main problem is a "rage" which robs her of any control over her actions and also induces a hulk-like physical transformation (though the change is quite likely less drastic).
Whether or not this rage is a result of Sonar's tampering or was the very thing Saber sought Sonar's assistance to remove is something I think I need Night Hunter to specify, although it seems logical that it was the latter of the two possibilities.
It may also be important to note that Saber harbors a dislike for the Maximal Elders.
Valkyrie (Venatrix):
Perhaps not as much a dark secret as a fact of her past (Only Primal know any details, and even then doesn't know the whole story, but still it doesn't seem quite so dark and guilt-inducing as it is mysterious), Valkyrie was the result of a specially designed robotic form created to house an ancient spark discovered by Maximal archeologists. The spark, and therefore, Valkyrie, was incredibly adept with processing information/problem-solving/general... thinking. Her body however will have sometimes have problems keeping up with her caclulations, and will not respond properly.
It is eventually planned for Valkyrie to become involved with Megatron and Tarantulas' Transmetal 2 experimentation. Her spark actually ends up mutating, along with her body, but she is destroyed in the process. However, the mutated spark will inhabit a new body which comes from the wreckage of her old one. This character will be Phoenix, and she will be a Predacon.
Venatrix (Venatrix):
Venatrix doesn't really have any hidden darkness. She has always been a Predacon. However, she did have experience working with Ravage on assassinations, and doing even some solo work. She was sent to inflitrate Megatron's group, but Dinobot found out her motives, and disarmed. Megatron gave her the choice to join their crew or die, and she chose to join the crew.
Given that this was to save her life, her loyalty is not, likely, very strong. Furthermore, she had joined to learn from Dinobot, and now that he has defected to the Maximals, her loyalty will probably be under additional. pressure.
She does hold to a code similar to Dinobot's (who she views as her mentor). This code, along with her lack of personal ties to Megatron, will likely be the main reasons that in "Code of Hero" she is intended to join forces with Dinobot, and from then on be with the Maximals.
Steelclaw (Blazemane):
Steelclaw will not defect to the Maximal side in the role play unless circumstances seem to dictate it necessary (not likely). But I figured I should bring him up because he does have a sort of darker past, or at least, unorthodox one.
Steelclaw was originally a Maximal government agent,but his missions gave him exposure to events going on on Cybertron, and he actually became sympathetic to the Predacons, seeing them as mistreated because of a preemptive fear of their similarity to the Decepticons of the past on the part of the Maximals.
Steelclaw defected, and worked with Megatron's crew to destabilize the Pax Cybertronia, joining the standing crew of the
Darksyde. The only manifestation this past will have in the current time of the role play is the retaining of a Maximal personality, which makes him quite docile in comparison to the other Predacons.
These are the secrets I can think of that belong to characters that have been currently accepted. If I forgot any character with a dark past, or you want to correct anything or add more detail, go ahead and do so.