Why sudden change in war boy's motives?

1. He had an incredibly simple world view and goal. He idolised Joe (see how excited he was when "He looked at me!!!" earlier in the chase. He knew he was dying, and wanted to go out in a glorious way (hoping also to impress Joe in the process). All his life, to him and the others around him, this seemed to be everything life was about - glory fighting on the road for Joe.
2. It all falls apart in an instant, leaving him a lost empty husk. He had a big chance to be the hero - and he failed catastrophically. Joe saw him fail, wrote him off as 'mediocre' (then shortly after, Joe's favourite wife is killed, adding guilt to his already crushing shame, taking away any hope he might have had of redeeming himself in Joe's eyes). The higher you are, the further you fall: in seconds, he's gone from thinking he was going to go out with even more glory than he'd ever thought possible, to feeling like everything he'd ever wanted is now impossible. He's failed and there's no going back. He's got nothing to live for, and can't even die the way he wanted to. He's at a loss, wracked with guilt and shame.
3. He makes a human connection and it gives him something new to live and die for. While he's at rock bottom, the red-haired wife finds him, shows him kindness of a kind he might have never experienced before, and they feel a connection of a kind they've probably both not experienced before. He doesn't know what this is, but he's got something to live and die for again.

I don't think it was seeing death that did it. He'd seen plenty of death, much of it far from glorious. Him seeing the heavily pregnant wife die took away any faint hope he might have had of redeeming himself in Joe's eyes, and added guilt to his despair - but the despair was already there.

Because he saw Splendid, Joe's heavily pregnant wife, get killed by the actions of Joe and the other War Boys, and immediately feels guilty for being part of her death. He tells this to Capable when she comes to check on him, and it's the main reason behind him changing sides. Also, factor in his desire to please Joe and 'be carried into Valhalla' by him, and the fact he fails and instantly loses Joes approval, and you can see how everything he thought he knew about his world suddenly changes. Plus, there's meant to be a glory in death for the Warboys (the idea of witnessing, the huffing of paint before they sacrifice themselves), yet when Nux see's a pregnant woman die, it's brutal and without heroism.

If Nux had been successful when he boards the rig, the sequence of events that caused Splendid (the heavily pregnant wife) to die wouldn't have happened. Although he still could have freed himself and tried to kill Furiosa, being responsible for the death of the Immortan's unborn child was far too great a failure to come back from. There was nothing but punishment waiting if he returned home, even if he killed Furiosa.

I too wonder at how relatively quickly Nux switches sides and goes to war against his society, but I think if you take into account how completely lost and goalless he is after shaming himself in front of his god, it is believable that he jumps at the new purpose offered to him by Capable (the red-haired wife).