Hexxus is the ancient spirit of destruction and all that is toxic to nature. As such, his only goal is to cause destruction, decay and death in any way he can.
Hexxus, true to his nature, could very much be considered the dark side of nature itself. At the core he relishes destruction and decay. He also shows to take great pleasure in the suffering and death of living creatures. However he also shows great fear when the situation turns against him, such as when Zak turned off the leveler and then when he was imprisoned once again.
He is an exeptional manipulator, able to influence others from the shadows. Hexxus also seems to show some admiration for humans, describing them as "wonderful creatures, so clever and useful". Possibly because he and humans share much in common , they both cause destruction to nature and its creatures. Nevertheless, he only sees them as a means to an end and, like all other living creatures, Hexxus takes great pleasure in their deaths just the same. He also has shown overconfidence, and enjoys taunting his prey, however his own arrogance is ultimately his greatest flaw.
As the spirit of destruction, Hexxus wields as variety of abilities and powers. He can change his form, which presumably depends on what type of poison or toxin he's currently feeding on, for example:Zak's spray paint, the smog produced by the Leveler, the oil in the Leveler, ect.
He has been shown to be able to mimic seemingly any voice. He is also able to breath fire, both in his normal and demonic forms. Hexxus can also possess machines, such as when he merges with the Leveler and also takes control of it once he scares off it's human operators. As stated by Magi Lune, Hexxus can rain down poison which presumably is his deadlest ability, for Magi states that during his first rampage the forest was "nearly destroyed and many lives were lost".
Aside from those abilities, Hexxus also seems to have some level of omniscience or omnipresence. Because as is hinted many times in his his song Toxic Love, it seems that, despite being imprisoned for countless years, Hexxus was still fully aware of how much humans and civilization had evolved over the millennia. Thus he, at least partially, understood the concept of machines, human greed, human work(shifts, breaks and all) and was aware of human things like shopping malls, cocktails, ect. Furthermore, he apparently also knew both the voice and even the personality of the lumberjacks' boss when he mimicked him over the intercom. The fact that he is a spirit futher adds plause to this theory. Indeed, Hexxus himself has said that he "sees the world and all the creatures in it".