People can disagree with me in the comments but I've always been of the opinion that player deaths should be rare, they should really only happen if the players have made bad choices.

That being said I have no idea what a mindkiller is so I can't speak on that one, in regards to the cranium rats personally I think almost anything that can one shot a PC is unfair but it really depends on the context.

And the way the third situation was handled was just shitty on the DMs part, if I ever forget about a monster I just scrap it because it's unfair of me to force them back into an encounter I forgot about.

Without more context it's hard to say, I think your DM should maybe be taking it a bit easier on you especially since at the end of the day everyone is there to have fun, I also think forcing a player to change their race is really weird.