Er, that was a rather psychotic portrayal of Superman...Like Superman 2 psychotic...
It makes more sense in context, specifically this is shell-shocked PTSD Superman that has dealt with years of all-out war against Lex and his cronies and is now at the breaking point. Then returns to find all his friends dead or dying.
Basically it illustrates even Superman can be pushed too far. Good alternate refference is near the end of the animated Superman series when he fights Darkseid, "All my life I have had to hold back, but not with you. I can let loose becasue you can take it."
So this is basically Supes letting loose.
Turning the subject to the American public and animation. Disney, WB, Pixar, Dreamworks, and countless others have unfortunately perpetuated the steryotype in American culture that cartoon=kids. Other parts of the world recognize animation as a valid artistic medium for cinema. Now that is not to say that these company's produce bad works, quite the contrary I really enjoy many movies from them, but they are set in the known-proffit media of producing family friendly programing.
A personal anecdote, when I saw the South Park movie years ago in the theater they actually had ushers at the door to the screening room warning people bringing in kids that though this was an animated movie it was -not- a child-friendly movie and I -still- saw -dozens- of small 8-9 year old children in there with clueless parents looking horrified at what they had taken their kids to see. It will be a long time before America accepts cartoons can be for grownups too.