Bertha was insane and unfaithful, too. Not that it necessarily justifies Rochester's infidelity...except by the time we see her, she's so far gone that I don't really blame him for seeking companionship elsewhere.

Actually, all things considered, Bertha is quite well-treated. She's not abused, he sees to it that she is housed in the more healthful environment of Thornfield rather than the potentially dangerous Ferndean, he hires someone to look after her (of course, she's a drunk...), and considering that the other option would've been a nineteenth-century asylum? Yeah.

By modern standards, of course, it's appalling, and I don't excuse Rochester for his behavior. But if you look at the options for the period, he was actually doing pretty well by her.