what is deserving of negative feedback?

Recently in a game, I noticed a strong player beginning to dominate. I asked for an alliance in game. It was accepted, although the allying player did not leave any borders undefended and could not have broken my continent.
The next turn I found myself with cards and an opportunity to eliminate the allying player for his. I did so. I felt, at the time, that elminating a player for his cards was the best chance I had of beating the strongest player.
The allying player stated that what I did was deserving of negative feedback. Since there was no unprotected border, and I eliminated all his armies regardless, do i deserve such feedback? Or was it just a very dastardly thing to do?
Also, apart from being a multi, deadbeating, and vendetta suiciding, in your opinions, what does deserve negative feedback?
The next turn I found myself with cards and an opportunity to eliminate the allying player for his. I did so. I felt, at the time, that elminating a player for his cards was the best chance I had of beating the strongest player.
The allying player stated that what I did was deserving of negative feedback. Since there was no unprotected border, and I eliminated all his armies regardless, do i deserve such feedback? Or was it just a very dastardly thing to do?
Also, apart from being a multi, deadbeating, and vendetta suiciding, in your opinions, what does deserve negative feedback?