The spawn camping thing is difficult. It really depends on game structure as Shelled pants has mentioned. However; a responsible group of players can back off to keep the game rolling. I've been to large events where the enemy team surrendered and quit because they were backed into a corner. I guess some games that may be desirable to smash your opponents, but if they quit you'll have no one to fight.
Mercy is a mercy. You don't "have" to take it. If taking a mercy was mandatory, players would try to use mercy during inappropriately times. A true mercy is simply this; you have a player "dead to rights", usually at extreme close range. You are then either saving him from getting shot close range, or quietly doing away with him. When you go to mercy an individual, you "should"; have him dead, target has no chance of escape or retaliation, and be ready to fire. If they don't take it immediately, "shoot them".
This is a general idea, and personal experience. I do not endorse anyone breaking the rules at a private field. Follow the rules, as you are a guest.
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