I strongly, strongly dislike The Governor, but I disagree completely that 1) he is a psychopath and 2) the character is flat. And I can't believe I'm about to defend him...
We've seen from the last two episodes that he is capable of fierce, deep and abiding love; even if it is unhealthy and possessive. After he gunned down the Woodburians, he became apathetic to his own life. He didn't dispatch the walker that came for him in the camp, he let it get almost close enough to rip his face off. After Martinez and Shumpert abandoned ship, he wandered just waiting for the inevitable. Then he met Lilly & Megan's family. He found his way back from a passivity with regard to death to actively caring about the well-being of other people and risking his life to provide meager assistance. Remember that he ventured alone into the nursing home to retrieve oxygen tanks for Lilly's dad? Remember that he tried to take them all back with him, but could only get two?
I think he really believed he had the opportunity to start over and truly didn't want to have the same power he had in Woodbury again... to begin with. That's why he killed Martinez, who offered to "share the crown" with him. While he was murdering Martinez he was repeating "I don't want it, I don't want it." Was it completely crazy to kill someone for offering to share power with you? Yeah, but if you consider that he believes a woman murdered his child in front of him, failed spectacularly in taking on the prison, massacred his own people in a fit of anger... yeah, I get that this crazy reaction is maybe not indicative of long term mental health issues. His reaction to Martinez isn't so much "I don't want to share" as it is "I don't want to become that monster again." I think being faced with the dead soldiers wearing the signs describing their wrong-doings (which were all things the Governor had also done!) was probably making him a little sensitive.
He tried to leave when he realized the camp was vulnerable and he was backsliding back into his old ways. The mud zombies were a heavy-handed metaphor to demonstrate the fact that he feels trapped and unable to make any move but murdery, coups to get power and stay safe.
In the end, he's murdered Pete because Pete stood between him and leadership, but I don't think he wants to lead for the sake of leading. He's a narcissist, that much is true, and he believes only at his direction will he be able to pool the resources of this group to protect the things he cares about.
Regarding the zombie aquariums, though, that's just plain crazy/a plot device to let the audience know HE'S BAAAAAAAAAACK.
His inability to coexist with the prison will be his undoing, and his inability to maintain his humanity. Again, as the whole Carol situation demonstrated, you can't lose your humanity protecting what you love. A monster doesn't get to have a family or love if being a monster is all they know or they can do.
I guess the reason I dislike him so much again, given the fact that they have developed him and the fact that they have shown he has some kind of backwards remorse, is the fact that he's so damn weak and just fell back to the old, failed way of doing things. He could be more than he's willing to be. And he's picking on my friends who just want to eat peas and sing to babies and try not to die of the ebola flu.