I'm a huge overlord fan and tensura fan and actually have read all novels of both (though i prefer overlord and have reread them multiple times) and i can say that i agree as well.

1) It isn't in Rimuru's style to do things, also he benefits more in the long run keeping her alive compared to her being dead (Farmenas kingdom follow up).

2)Rimuru values the lives of his friends and also those that they would care for. Thus he values their state of mind, caring to those that are worth doing so

And exactly, Ainz is not a mindless killing machine, the author describes him as a dumb protag but he does put a lot of thoughts into what he does. Rimuru can lash out sometimes because he still feels somewhat human (when becoming demon lord id say less but you can def see that he still keeps his japanese identity af, like at the festival) while Ainz has lost his humanity.

There is one thing id disagree and that it is that Ainz warns his enemies, thats so far from the truth. I mean you could back it up with the war declarations that the SK has done in the past, but it isnt comparable when talking about fights IRL like the harvest festival with Rimuru. In those cases Ainz takes a lot of precautions and often will leave out information for his enemies, no warnings whatsoever, just certain death.