Why are they aggressive, destructive or cruel?

The death instinct is directed against the organism itself and thus is a self-destructive drive, or it is directed outward, and in this case tends to destroy others ranter than oneself. When blended with sexuality, the death instinct is transformed into more harmless impulses expressed in sadism and masochism.

Lorenz states that "present-day civilized man suffers from insufficient discharge of his aggressive drive." Both, by different routes, arrive at a picture of man in which aggressive-destructive energy is continuously produced, and very difficult, if not impossible in the long run to control. The so-called evil in animals becomes a real evil in man, even though according to Lorenz its roots are not evil.

--Erich Fromm

Why are they aggressive, destructive or cruel?

So-called polar disease, also known as expedition choler, attacks small groups of men who are completely dependent on one another and are thus prevented from quarreling with strangers or people outside their own circle or friends. From this it will be clear that the damming up of aggression will be more dangerous, the better the members of the group know, understand, and like each other. In such a situation, as I know from personal experience, all aggression and intra-specific fight behavior undergo an extreme lowering of their threshold values. Subjectively this is expressed by the fact that one reacts to small mannerisms of one's best friends - such as the way in which they clear their throats or sneeze - in a way that would normally be adequate only if one had been hit by a drunkard.

--K. Lorenz