Joel is constantly reincarnated with all his memories. He can do this due to The Power of Love. The Power of Love is the only magic he believes in and the only magic which can affect him. The watchers have no sway over this man. But they can affect his allies and teammates.
However Lizzie is also constantly reincarnated. She doesn’t retain her memories though. This is due to her godly status, not the power of love, so it’s different to Joel. She doesn’t always reincarnate as a person.
Joel’s goal is always to find his wife once more.
His mania is just a side effect of living countless lives with all the memories of each. His ‘red madness’ at the end of each life series is due to this. He’s either lost his wife or hasn’t been able to find her in this world.
And the world is dying. The worlds of the life series are dying worlds, they are made for a short period of time and then they are supposed to die, with the last cast member.
And he loses hope, so might as well get out with a bang. Because then in the next world he might be able to find his wife once more.