Sure it wasn't out of altruism. If Ukraine loses, more things will happen (remember, this didn't start with Ukraine, we had Chechen, we had Georgians, we had Ukrainian Crimea and now Ukraine)

1. Ultimately we'll have war in Europe which will force US to be involved (as mentioned Ukraine is not first and not last)
2. More likely invasion of Taiwan (if US couldn't help Ukraine, how will they help Taiwan?)
3. We will get a nuclear proliferation, a bunch of other countries (Japan, South Korea, Poland, Germany and others) will be interested to obtain nukes, as that will be the only guarantee to not get invaded