Yeah, that was pretty stupid. It's not very helpful or constructive to say "this sucks, don't listen to it."
You "hinted him to our general acceptance?" What bearing does our "general acceptance" have on what is good and what is not? So what if he enjoys Killswitch Engage? So what if it's not of the general opinion of the board that that is not good music? What good does "kill yourself" do to either showing him that you feel that there is better music out there or really anything in general except demonstrating the fact that you feel that you have superior taste?