Why do the horde leaders kill and/or severely "punish" their own allies?

[Ashleg]

I don't mean the vermin who are plotting behind their backs, I'm talking about those unfortunate guys who happen to be in the wrong place at the wrong time.
Like that rat Cluny made an example of in the graveyard (forget his name) by whipping him with his tail repeatedly, and just others throughout the books that perhaps said something the leader didn't like and were killed for it.

[James Gryphon]

Well, a lot of it is about maintaining fear, which is a key part of most of the warlords' leadership styles. On a basic level it motivates their soldiers to work harder and to be as obedient as possible (so as not to be like the unlucky sap that got done in). Also, if they made comments the warlord disapproved of, it 1) stops those comments, and 2) clearly shows the consequences that will come from following that line of thought, an object lesson that few hordebeasts would fail to take to heart.

On a deeper level, though, it is also a twisted way of getting respect from veteran soldiers. Fighting vermin look on traits like pity and mercy as weaknesses, and admire those who can do the most damage. When Cluny or somebody kills a beast for a minor offense, it shows them how 'hardcore' he is. It reminds them of what a great fighter they have leading their group, and reinforces the idea that he will be able to do this to any beast that stands in the way of the horde's profit.

Now, it clearly isn't a very efficient technique in terms of manpower, but the horde leader is betting that the increase in motivation among the rest will offset the loss of one creature.

[Ashleg]

Wow, that explains a lot. Thanks!

[Flib Bigboat]

I agree with James on that vermin leaders kill their own soldiers to teach the other soldiers that they are to be feared and respected, but some of them like to do it for fun. Or when something goes wrong, such as a battle plan, and they take it out on random soldiers, using any excuse. Like in Marlfox, when a rat dropped his shield, and the High Queen Silth had him thrown into the lake, where he was killed by pike. Some other rats were grumbling about it later, and they said she was just looking for somebeast to punish because she couldn't get rid of a ghost. (Sorry for the Marlfox spoilers!)
Doing things like this usually keeps the soldiers in line, but sometimes it has an opposite effect, and turns the leader's army against him, which creates more problems. Then again, killing random soldiers sometimes ends up with the other soldiers hating their leader, but they're too afraid of what the consequences will be if they rebel against him. It's not a good idea, and there should be some other way for a warlord to get his army to respect him.
Just my two cents.

[Sebias of Redwall]

Agreed. Good points. (I don't think you have to, but if you are concerned about giving spoilers I'd maybe give the warning beforehand. )