This is not a yes or no question, but I will say this is not necessarily something that I would do. Did he have Right Action? It is a bit beyond me to say that, however, killing - and even killing yourself - is against what Buddhism teaches. So on the surface it appears to be a wrong action.
However, did he have the intention to kill? Or did he have an intention that was different to that? His intention, if you know enough about it, was to try to stop the difficulties that the Buddhist people were experiencing in South Vietnam. It wasn't necessarily against the war, if you know about the history, although that was tied in with it. According to intention, perhaps the intention of the Monk was based in a good direction. Was it Right Action? We can say no, it is against the Buddhist teaching. Yet on another level, what did he produce as an end result? It actually was a great deal as it brought attention to the difficulties that were going on in Vietnam. This attention eventually stopped the war. Was he responsible in that way for helping stop a war? If we're looking from that direction, then what he did was instrumental and helpful in stopping a war, a war that many people believed was not a just war.
Was he balanced in himself with his Compassion and Equanimity? That's another question. That's different from how we would look at it from the outside. Was he actually balanced within? Was he willing to sacrifice his life in a compassionate way, and not actually care in an equanimity way? That seems to be the approach that he probably took. Having never met him, and discussed directly with him, it's hard to answer this question directly with yes or no. But I think you kind of get the feeling that I'm working at - it depends on how we view it and what his intentions were. Based on different ways of looking at what he did, there was some benefit. Based on looking at it from another way, it was killing oneself, and Buddhism is definitely against that one aspect. But was it more a personal sacrifice to help safe others rather than killing oneself through a sense of aversion to himself and the world?