Why would you judge God any differently for doing something similar on a much greater scale?
The flaw in that reasoning is that humans are NOT God - not even close. If you believe that God is much greater, both in terms of power but also in understanding and ability to 'read' the heart, mind and motivation of people, then it's no comparison.
The problem with the people you mention - Nazis, 'radicalised' religious people, etc - is that they arrogantly elevate themselves to God's level (knowingly or unknowingly), thinking they have the RIGHT to take lives as they see fit. And in the case of those who think they are doing 'God's work' (such as that shooter, maybe), they compound the error by presuming to act 'for God'.
However, humans do not have the right to 'play God' in that way, precisely because we are just bags of water and meat, or 'mere grass that withers' etc, as the Bible puts it. We do not know what goes on in the mind and heart of another person so cannot judge them with total certainty, and we definitely cannot restore life to someone if we take it away in 'error'.
TonusOH put a finger on one crucial difference that entitles God to give - or remove - life: He made it, can remake it, and can give it back to whomever he chooses. He 'owns' the very concept of life - we do not.
Couple that with God being able to read the hearts and motivations of others, and to even foresee the inclination of people, (which no mere human can do with certainty) means that comparing God's right to kill with humans is comparing apples with oranges.
Such belief is a stepping block upon which more radicalized beliefs can be built: "If it is God's will to do it eventually and everyone agrees with it, than I could do it now and I'd be doing God's will. Now or later, what's the difference?"
That may be how such extremists think, but it should be obvious to everyone else how flawed that thinking is. It's not just about "now or later", it's about WHO does the killing (and why). Some extreme religions are more problematic with this because their 'holy' texts can be interpreted to appear to permit their members to do it (I'm thinking Islam, for example) but certainly for supposed Christians, it is clear from the NT that they should not be killing anyone themselves.