He could have played Kingmaker, but then he would have had to make a political choice. I don't think he saw good alternatives and more importantly, I don't think he wanted to get into politics. Putting himself as the leader of a government would have made him a military dictator. I don't think he wanted that, either. He was willing to play ball if everybody else carried their weight, but everyone was looking at their own interests.
So, I don't think he was selfish. On the contrary, even. However, whether it was a good idea to go, is another question which I find more difficult to answer. It is nuanced, which is great about it. In the end, he followed his own moral compass, which was made of a lot more honorable stuff than the House Lords who plunged the Inner Sphere into war.
Also, I think it is important to consider that after so many wars and years of service, he was probably reaching "War never changes/stops" kind of position, so he suspected they were damned if they do, damned if they don't, so he opted out. You could say that there is some selfishness in that, but I don't think it is purely that, when he is also doing that to save lives and worlds by removing a big military from the picture (which was, it is very important to note, hemorrhaging units to the Houses).