While I think the shirt is funny and should be worn proudly, I can see how someone might be offended for non-racist reasons. Some people might simply find the depiction of death to be offensive.
That said, it's entirely possible the offended person was a raging racist.
Edit:
This edit is for those of you who are saying that people shouldn't be offended by violence since it's on TV. I think you're missing the fact that the point entirely.
The fact that a depiction of violence is making a valid point doesn't make it less offensive. In fact, that would detract from the message. Sometimes it is important to make a statement using depictions of violence to get a point across because the violence is offensive.
That said, I think it is perfectly reasonable for someone to find this shirt objectionable for non-racist reasons. For example, many people prefer not to expose their kids to depictions of violence. I think it's this person's right to wear the shirt to make their point but the consequence of wearing it is that some people may say that they find it offensive for both non-racist and racist reasons.