... I think I've also seen media reports that Rosenbaum had actually used force against his victims, so I don't really consider him a child lover, though he may have been a pedophile - or, like you say, he may have simply been targeting those who could not resist him.
As far as I'm concerned the world is better off without him.
The other aggressors may have acted under the false assumption that Kyle had murdered Rosenbaum, rather than defended his life from Rosenbaum, and so if Huber had lived his actions might have been excusable as those of someone trying to prevent harm - but in effect they were all endangering Kyle's life when he shot them - and all had histories of violence.
In light of all this I can see some sympathy for Huber and Grosskreutz, but not for Rosenbaum.