With age I came to conclusion that anything making killing of any kind even remotely fun is utterly wrong. It might be necessary with dangerous insects, but it should not be fun or recreational.
It is a hard ethical dilemma and any line you draw will be arbitrary. Some people hunt rhinos, some catch fish or shoot birds as past-time, while others get sense of satisfaction from gunning down insects. I still condone pest control, but I draw the line at reasons like "for fun", or "because I'm bored", or "to feel more powerful". The intention/reason is important, because it affects the future. If you do it for fun, you have no reason to ever stop or to consider alternatives once they are made possible.
To answer another post, I think killing in video games is completely OK and something I often enjoy doing.
I'm also enjoying the tone you've set for discussion and I'd like to hear your personal views.
Essentially, my stance is that there is a difference between accepting an action as necessary and engaging with it as needed and making said action fun / associating it with enjoyment. I don't count video games as part of the issue for the moment since, at least currently, it does not seem like the characters and units that populate video games are in any sense alive. This may change, however, and if it does then I expect radical consequences will unfold.
If you find it fun to use or repugnant that's a personal thing.
Flies are yucky pests that I don't want to have around.
All of the articles (e.g. [0]) I found in a quick search in response to this citation-free assertion suggested that the answer is more nuanced that that. I'd also observe that higher primates (e.g. chimps) that have pain mechanisms that are biologically very similar to ours can have a very different response to us. If you watch film of social interactions of chimps, e.g. [1], it's a fairly routine occurrence for an individual to apparently ignore injuries that would have a human screaming (e.g. a finger partially severed in a fight with a rival), especially if it would affect their hierarchical status to run away. So I wouldn't read too much into observed behaviour as an indicator of what is actually going on, at least for primates.
I have yet to meet the Jain who has studied cellular biology.