First, the evidence that Assad actually gassed people is extremely weak at best. More likely is the Sauds and Qataris funneled more chem weapons to ISIS, which then leaked out during combat due to bad handling. Parse the news reports closely for actual verified facts instead of opinions treated as facts, and you will quickly find that the whole incident has basically been twisted beyond recognition due to the wishes of some major media houses to push clickbait and foment war.
Second, even if he did - so what? Like what, 50 people died? Terrible to be sure, but a drop in the bucket. The ISIS forces that Trump is now supporting have killed an awful lot more Christians than that, and raped orders of magnitudes more. Heck, the US itself seems to have accidentally killed four times as many civilians in an airstrike just a couple weeks ago.
Third, there is no thought that Assad poses any threat to us. None. Zilch. Zero. For a President to make war without Congressional approval is unconstitutional, unless they are responding to some imminent threat or circumstance.
Sure, Assad is a bad, terrible person, yadda yadda. So what. While we certainly sure shouldnt support him, theres also no good reason to become ISIS's air force either.