Firstly, cry. Ok, just kidding- don't cry-haha!
You could say that he's presented as the nastiest of the nastiest. He, like Slim, is shown as the extreme. Whist Slim is what Steinbeck may be portraying Slim as the 'ideal' ranch hand, Carlson is exactly what Steinbeck is suggesting people should NOT end up like.
You could argue that his moral intentions are right, however, as he wants to put Candy's dog "out of his misery". It seems really terrible, but if you think about it based on the dog's description of "moth-eaten" and having "pale, blind old eyes", it actually seems like the right thing to do!
Finally, you could describe him as lacking understanding, and being overall callous. This is reallllllllyyy shown at the end, in "now what the hell ya suppose is eatin' them to guys?". Wow, he lacks emotion!
The fact that this is the last line of the whole novella shows how significant a character Carlson is, and that he, like Crooks, is put in the novella as a message. Steinbeck might be saying: this is what we don't want men to end up as, which is structurally, in this case, really fitting, as the end of the novella literally 'ends up' in such a way.