Due to the limited numbers of certain high end creatures that spawn (such as dire wolves, bulls, hell beasts, and giant toads), it is best that you kill whatever you tame. Why? If you kill a creature you just tamed (that you can no longer get skill for, so why keep it?), a new one will spawn (that you can gain skill on!). Isn't that nice? This works with all creatures, not just the high end ones I mentioned. Also, a creature can be tamed only so many times before no one can tame it again for skill. These pitiful creatures need to be put out of their misery.
Many people complain that killing tames is too hard or costs too much money. I disagree. I have been taming since spring/summer 1999 with 2 different tamers. It all depends on how ingenious you are in killing them. So how can you kill your tames?
Even a low level tamer can tame a bear or panther to kill their tames with. Polar bears and grizzly bears work the best. Your pets also make good protection from monsters that might be in the area
If you are a skilled warrior, killing your tames should be no problem with the weapon you chose to excel in.
Non-aggressive creatures will auto attack any aggressive creature in the area. Just lure the monsters near your tamed creatures.
True, blade spirits only attack very dexterious creatures, but you can use them to kill your tames. Remember, non-aggressive released creatures will attack aggressive/red creatures on sight. This is very efficient at arctic isle, due to the sheer numbers of released creatures there. Remember, blade spirits are highly unpredictable and may attack innocents, making you grey (Felucca only). Make sure you are as proficient in the dispel spell as you are in the blade spirit spell.
If you are also a proficient bard, you can make good use of your instruments. Just provoke your released creatures onto the strongest thing you can find (such as strong monsters or grizzly bears).
Peacemaking for killing tames is most effective when working around other aggressive creatures. When you tame your creature, make it guard you, then peacemake. Depending on how close the tame is to the monsters, they should retarget your tame. If they retarget you, the guard command will kick in and they will attack whatever aggresses towards you. For this method to work, do NOT release your tames. If you are going to do this, make sure whatever is attacking your tames is plenty strong enough to kill them.
If you opt for the tamer/mage route, your tames can be an excellent target for your magery. It is possible to gm magery with just flamestrikes and gates. I almost reached gm through normal game play with just those spells.
If you are nearing grandmastery in magery, you might opt to summon daemons to kill your tames. Daemons are very efficient animal killers. Some elementals can be just as effective, and just a bit cheaper to cast. You're gonna summon all those creatures while working for gm magery, so why not put them to good use?