Clearly he states the reason why also since he said he has screen shots and recorded the logs he can obviously show you that you Mass RDMed people, if he can show it to you and see the evidence of you constantly killing people randomly in a row then obviously it is very suspicious and if it was over about 5 kills in a short amount of time it will highly likely to be mass RDM
If you wan't to be unbanned then you must have proof of you not doing so, no proof = no unban
Also please do not swear