The ==valid== reason for removing Assad is the EXACT same reason I was happy to personally involved in the Persian Gulf about a dozen years ago. In a volunteer military and volunteered on top of that, because I wanted to be involved.
Removing Saddam was, in and of itself, a good deed, because he was that bad.
Same for Assad.
Such a thing does not happen 'in and of itself'. It will part of larger events. Ok, so we stick around to give the 'good guys' a solid footing to build on......
.....and ten years of experience later, it has become clear that a 'strong man', even as nasty as Saddam, Assad, Khadafy, etc, is the best that can be hoped for (at least in the Muslim world. Germans, Japanese, etc, are different).
The best (SLIM) possibility that has any chance of happening after Assad, is an Assad 'clone' who is just as brutal, but whom we can say 'is not Assad'. Far more likely is ISIS or something like ISIS, that is far far far worse. We can hold that off by putting boots on the ground and spending blood and treasure, and then as soon as we leave, the horror story will continue as if we had never been there (heck the horror story won't even stop while we are there....).
The moral of the story:
When you have a Saddam, or Khadafy, or Assad, etc... smack em if they export ugliness, but otherwise leave them alone. In some cases you can even get them to rein in some small percentage of their evil, but if you think you can make everything better by taking them out, think again.