Usually, when a player lags badly, they appear to move slowly or stop entirely between packets, making it much easier for other players with lower pings to kill them.
In some instances when a player has particularly bad packet loss you might find this happens, but it's pretty rare.
If it happens then don't worry, I am almost certain that they wouldn't be able to kill you, either.
If you see one of these laggers and they have a better pinger near them, my suggestion is to take out the lower pinger first. Then just get in line with the lagger and line up a shot when they are running away or towards you.
It shouldn't create too much of a problem, really.
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