Im not really sure but I wouldn't jump into saying that Donovan is a good guy now
From this back story and how Melinda descript him before taken by alien, it's easy to assume he's simply "normal" person back then, whether it's his real personality or mask to hide something deeper we won't know unless this manga ended (since that's the original plot).
Forgot to add.. But one thing for sure his personality changed. It can be he simply it isn't necessary to hide it anymore or he's really just good guy but changed by some kinda experiments related to Gramps and Anya.
Damn so final boss actually just normal good person. Why I feel like his changing of personality related to Damian? Since Melinda mentioned he's her beloved but also curse, something about his pregnancy/birth probably led to Donovan to accept that project or something?