Was he really amnesiac?

I never for the second believed he was.

Jaime played very well and I paid a lot of attention to detail, mainly the eyes. This was when he gave away, when script told him to do so.

Even when waking up from the surgery, you could see in his eyes how he is looking around and thinking of the plan. Amnesia was best bet.

The huge giveaway was also the scene in hospital at night when he got up and stretched, this was him... the killer him.

In previous seasons it was shown because of his psychology degree, how he likes to manipulate people and enjoys it. Besides safe bet, the amnesia was a lot of fun for him.

He made a biggest mistake in underestimating Stella. Finding Rose and David were things he did not expect.

As for killing Bailey, I think he wanted him to be punished and also get the thrill for the one last time.

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I think he was definitely faking it. At first I thought he might have genuinely experienced amnesia, but after he had that long conversation with his nurse, when she left the room and said she'd pray for him, he laid down and the look on his face just told me that he was faking it. I don't know, I'd call it good acting on the part of the actor who played him, deliberately keeping it as ambiguous as possible. But from that point on I was convinced he was faking, and his behavior afterwards only confirmed it to me. Like when Stella and Anderson were talking to him about the murder in London in 2002 and the abuse in the orphan home, and he talked about how "things were starting to come back to him", conveniently just enough for him to continue his taunting of Stella, and then going on about how his murder history "intrigued him". It was such bullshit and it just showed he was faking, but that they had gotten under his skin. And his continued violent behavior, attacking Stella in the interrogation room, attacking Dr. Larson. Plus the London murder, which I believe he did commit and wasn't an accident. And there's all the evidence the doctors talked about, that the scans showed no brain damage that would explain the amnesia. The amnesia itself, too, was too convenient; he remembered everything perfectly from before 2006, and everything from after he woke up, but conveniently nothing from the already-known murder period. I'm no medical professional but I don't think amnesia can be quite so clear-cut between one point of time and another.

Finally he killed himself because he realized there was no way he was getting out of it now, he had slipped up and they were going to get him, so there was no point going on. It was also one last "fuck you" to Stella, part of his whole pattern both before and after his "amnesia". When they last talked, she said she didn't want him to cheat the system by dying. So that's just what he did. He remembered everything and knew what he was doing.