[Generally_Suspicious]

On this server, you can not KOS off of the primary weapon of anyone in the area. You can only KOS if you see someone killing another player or running away from an unID body that they clearly could not have missed.

The shotgun was just the cherry on top. Given his location, he was either the killer or had witnessed the kill and let the killer run past him which is traitorous.

This is unfortunately what is considered (loosely, or firmly) meta gaming.
Gathering information from deduction, assumption, or any otherwise means that is not firm and unchallenged (such as witnessing the kill or dna, for two main examples).
It is something that is not allowed on this particular server. As well most others for that matter from what I can tell.

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Also; curious about this mine-craft trick. How ever i'm not sure we should disclose that information openly for people to understand and abuse. Probably discuss it with a higher staff in private such as @Mr_Rippe , @DaKody , or @"Mr Turner". Probably specifically Rippe since he is part of the Dev team.

See my above response.

As for the minecraft trick, it's not really a bug or exploit but it may look like RDM to an outside observer. Basically, at the beginning of the game you go into the nether and take note of anyone who's in there. Then go back into the real world and watch the portal. If anyone comes through the portal into the real world and you didn't see them go in and they weren't in there when you first checked, it means they came through the traitor room into the nether.

There's a number of reasons any gullible or trusting individual would walk away from a kill. You'd be hard pressed to find anyone who isn't trigger happy. How ever a quick "They started it, I defended my self and finished it" can get you off the hook for a good minute or two.
That is borderline meta gaming. While yes the only other entrance is a T use only area; it's still high sus at best. Which is kos-able in over time how ever. 

Things like that you can how ever do are. Say there's you and 6 others left. 5 of those people are proven. You may now kill that other person. 
Say there's 2 non provens, you can in over time demand they fight each other under the penalty of high sus for not acting on it. Which again is kos in over time. 
Assumptions - not ok
Math - usually ok depending on how its gone about
What you're saying is really one of those things where it's all on how you explain it and execute it; whether you get away with it or not. 
If you did that at low pop times you might be fine. Try that with a full 32 person server and now we're all questioning how you possibly kept track of that many people when most of them are with out a doubt out of your sights.

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As for the slay. Try calling a detective for DNA. Much more accurate results; less risk of slay.