No its not offensive. But your teammate was being offensive. Now to break down what I mean let's look at it. The guy who blew himself up. If he would a gone 'murica after its funny. But the way you team then responded was well he takes the game to seriously and as such was intending offence. Now if he would a gone typical american it may have been funny if he used it with a light hearted voice or offensive if he said it with disdain. But the point in general it isn't offensive until someone is trying to be.