(...)
So going by your own words, from my point of view you started the whole thing.
The player was reacting to your intrusion, I do not blame them for that, also you hint at dagger, yes it is faster, a good way to beat someone and tick them off, as you butted in on his fun, As for calling his Guild mates, do not blame him, give you a bit of your own medicine.
The only thing I disprove of is their challenge and claim [a comon unwritten rule that if someone else was in the pit noone else should be there ]
But we all must remember there is two sides to a story, so do not make a decision based on solely one side.
Yes, there's always two sides to a story and we try to listen to both in every case but let's make one thing absolutely clear here: there was no intrusion here. Players do not have to ask for permission to attack a free mob and a free mob is any mob that is not already engaged in combat with another player.
It is polite to do that. It is appreciated and it really does make the game atmosphere better, but nobody owns spawns and there is no validity in said "unspoken rule". It can be considered rude to take a mob someone else is waiting for without asking to share, but if that happens, nobody has the right to resort to harassment. Likewise, if you are fighting a mob with lvl0 melee or in fully defensive mode just to make the other players wait forever you are harassing them.
Be mature about it and if you don't want to share or compete for the kill, go find another mob. It is a good idea to talk to the person and offer to share, if they are jerks about it you always have the option to call a GM, file a petition and /report. It's your choice to stay or leave but you should not get into a pissing contest or else you'll be the one breaking the rules and getting punished for it.
We GMs will be glad to step in and help but it is the players' responsibility to know the game policies and rules so not knowing them is no excuse. They are available for anyone to see so don't go around making up new ones and for God's sake don't call your guildmates to gang up on other players! That will only get them in trouble too.