He was the dark side of Bob's self, all the hurt and anger that was inside him. The scientists that were working for Val (Julia Louis-Dreyfus) experimented on him to try to make a super-being out of him, but they released the dark side (The Void) which took him over
He managed to suppress it, but Val manipulated him and it ended up coming out again. Its darkness was so powerful that it sucked other people, including the other Thunderbolts, into itself and forced them to relive their darkest moments. The other Thunderbolts found Bob, and with their companionship they helped him confront the Void and he released everybody
But he's not all better, the Void is still in there. He's just learning to deal with his dark side. Like we all must