I know that badges can be added using some queries , but I feel it’d be good if there are some badges by defualt for users who have accepted flag often, as they help admins keep everything well-organized and spam-free.
e.g. those who have 10 accepted flag, those who have 25 accepted flag, … .
No one flags anything in my communities, so these badges would not be useful to me. That’s my personal “first check” for a default. 
Right but his underlying idea is correct – if we encourage more flagging maybe it will happen more?
I see it as a balance between: discovery of a useful feature, and encouraging others to flag.
I consider flagging to be a negative action most times (meaning it’s a reaction to something adverse, rather than a proactive step on the badge-reciever’s part), but I’m open to thoughts around that. I just don’t want folks “looking” to flag, ya know? But again, not a problem I’m solving. 
I think you are both right is what I am saying 
@Pad_Pors, I see what you’re thinking–you’d like users to take more personal responsibility for community standards and more personal reflection on their own behavior, where the badge is a visual reminder. 
My thought is that singling out the specific helpful community behavior of “flagging” might increase the frequency of false flagging, as well as mod workload.
I know there are a ton of badges, but I’m not familiar with all of them or how they can be customized. Are there any that more vaguely speak to keeping the community on track? Maybe a “Civility” badge (as in the auto-message, “Thanks for keeping our community civil”)?
10 and 100 “helpful” / “agreed” flags would be a good starting point for more badges. If you want to get more specific, we could do things like 10 agreed spam: “Spam Fighter”
Flagging is a difficult scenario. Nobody wants to feel like “a rat”. 
that’s why it’s better if only accepted flags are being awarded, and not all the flags.
not a rat, but a criticizer or even a reviewer. we have changed the name, the icon, and the auto-messages of flags in our community toward the criticize meaning, and it works quite well. from that point people want to have a contribution toward making the conversations better.
afterward, users even flag good topics but with bad (unclear) subjects, and this helps our admins find those topics which need moderation in topic title much faster.
I agree about awarding the badges based only on accepted flags, but mods might still have to wade through increased false (and legitimate) flags, to determine which flags are accepted in the first place. 