Alright, time for some updates. I see I did something stupid, and I apologize. I see now that I was kind of being a prick. I regret what I did, and I would apologize to the deviant, if they had not blocked me. I'm closing the thread ASAP.
Telling them they need to improve is nice and all, but it's really unhelpful if you don't elaborate on what they need to improve on and ways they can do so. Considering you didn't seem to say anything else to them within that comment (that I am aware of, anyway), it really only sounds like a sugar-coated "Your art sucks."
If they ask for critique, and you give them one but they reject your advice, then that's on them and they won't grow as an artist. It might have been more helpful (and maybe they would have been less butthurt) if you mentioned ways for them to improve or areas they needed to work on. That would be more of a critique than just telling them they need to improve.
Dude, you just went on his page and said "You need to improve" when he didn't ask for critique or anything (by the way, telling him "You need to improve" isn't fucking critique)? And you're surprised that he came on your page and was butthurt about it? His response, no matter how butthurt it was, is warranted.