Since there must've been commercial pilots, doctors, bus drivers, nuclear power plant workers, etc. who's deaths would have caused more death, in essence Thanos is responsible for even more deaths.
Even if his genocide was justified, his recklessness wasn't. He didn't seem to have much concern for the soon to be 'saved' half of the universe while we was in pursuit of the infinity stones and destroying cities and planets.
There were plenty of other ways to save the universe.
He saved half the universe.