In a recent episode of the cartoon Family Guy ("Petarded"; aired June 19 2005), Peter Griffin is so convinced that he's a genius, he takes an IQ test, to find that he is actually mildly retarded. In typical Family Guy style, the world ridiculously overreacts, treating Peter as if he were severely retarded, and incapable of living independently.
Peter then gets in an accident with Tom Tucker, the newscaster, who recognizes Peter as that retarded guy, and so Tom excuses the accident. Thus, Peter realizes that he can get away with a lot of inappropriate behavior, just by saying he's retarded. One scene shows him methodically kicking in the stall doors in a women's bathroom, each time saying "Retarded," and each woman excusing the behavior.
Keep that episode in mind the next time you hear lawyers and pundits arguing that a convicted murderer should be spared the death penalty because the results of an IQ test show he's mildly retarded.
It doesn't take much intelligence to understand the concept that killing people is wrong. There are a lot of dumb people in the world, but they don't go around killing. About the only excuse I accept to exempt someone from the system of justice is serious mental illness (the person doesn't realize they're killing someone) or serious retardation (the person is incapable of realizing the consequence of their action). In each case, such a person should be institutionalized for life.
If you interact with a person and he doesn't strike you as retarded, then he probably has the capability of knowing killing is wrong, and low intelligence is no excuse. It may be funny within the mild mischief of a cartoon, but it's a deadly serious issue in real life.