Dude, do you know where you are? Yes, slaughtering """defenseless""" oppressors is justified, not the least of which because there's no other feasible way to get rid of them as a class. Ending slavery in the South didn't end the rule of the now-former slaveowners. I suppose Mao might have tried to "reform" the Tibetan ruling class – apparently he much prefered "re-education" over execution – but they din't hav that luxury. If I could hazard a guess, it's simply because they were too damn awful.
Just trying to puppeteer them with threats of invasion isn't really feasible, because it'll leave the conflict open to the entire world to directly butt in, and you can bet the capitalist countries would sanctify those damn monks. Even more than they already do, I mean. Not to mention there's a limit to how much they'll obey without a show of force first. Lastly, well, they deserved it.
I'd just like to point out that numbers about Tibetan population are notoriouly unreliable. The only group who knows for sure is the Chinese government itself, and I doubt the numbers they release to the public are truthful. But every single other source gives only estimations and almost all are tainted with propaganda too, with the government-in-exile providing particularly distant figures from China's. Every once a a while, the goverment-in-exile say the Chinese are swamping Tibet with ethnic Han, but that's yet to be echoed by other sources. The number of Han in Tibet is even more unknown than the number of Tibetans. Quite frankly, I don't know what's the most trustworthy source for Tibetan demographics, but there's no widely accepted evidence to show there has been genocide, either by violence or by colonization.
But I can't deny that it's imperialism, yes. Commies most definitely weren't free of that sin. Tibet was an ancient country that had sparred with China in times past, until the latter became the undisputed bigger power and Tibet, like Mongolia, was annexed by the last Chinese imperial dynasty over some 200 years ago. As the empire collapsed, both regions declared independence in 1911-12, but as soon as Mao re-unified China, he went after what was very arguably a "historic region of China". Pure bullshit, of course. A funny thing is that China would probably have invaded and annexed Mongolia too, had the USSR not taken it under their wing before. Regardless, to the best of my knowledge, neither Tibet nor Mongolia have any particularly valuable resources. The annexation of Tibet was just old imperialism I'm afraid, with the peculiar exception that life there was such shit that a foreign invasion actually massively improved the local quality of life. It's one of the rare cases where imperialism definitely was a net positive for the invaded.
Fun fact: one of the people tasked with planning the Tibetan invasion was our old buddy Deng Xiaoping.