Do you ever have guests that look like this? Everything is taken care of for them, and for some reason, they are still angry? Well, if this happens, that means they are idiots. To "take care" of this, you can either get rid of the guest, or try to make it happier. The easiest way to "take care" of the situation is to get rid of the guest. To do this, you can either kick the guest out of the zoo, or kill it. I always try to kick them out first (I'm not an evil person ;-). To do this, move the guest to the zoo entrance. Most likely, they will leave, but if they are even bigger idiots, they will go back into the zoo, lowering your average guest happiness. If this happens, you can try to make them happier. If you have a large piece of land that is unused, you can move the guest into the middle of nowhere. By the time it gets back to the active part of the zoo, the guest will be much happier. If this still doesn't work, then you can either deal with it, or kill the guest. I always try to avoid killing the guest, so I usually just deal with it. Dealing with it will usually either give the guest time to get out of the zoo, or stay in the zoo and become happier. But if everything fails, you might just have to kill the guest. You can do this two ways; by drowning it or feeding it to a hungry dinosaur. Drowning the guest is much easier than feeding it to a dinosaur, since dinosaurs only eat guests when the dinosaur has escaped, which is not good. To drown a guest, put lots of decorative fencing in one area (the middle of nowhere would be nice), and fill the area inside the fencing with water. Then move the guest into the "pool" you've set up, and it will eventully drown. However, there are some guests who are pure Houdinis, and they can escape from the "pool". But if you have enough fencing, this will not happen.
In one sentence: If a guest is angry for no reason at all, first try escorting it out of the zoo, then try moving him into the middle of nowhere, then try dealing with it, before trying to kill the guest.