It's probably something along the same lines how we breed animals or cross-pollinate various plants. Lavos is aware that it can mix two things together, or twist the genetic makeup of what's there, and does so to get the best of both worlds, so to speak. For example, say one was to cross-pollinate a big boy tomato, a tomato that gets up to about two pounds, with an early girl tomato, which ripens about a month before any other tomato and tends to be hardier than the others as well. You'd come out with something that would produce sooner than most, and would also be great in size. Sure, it wouldn't be as big as the big boy or come as soon as the early girl, but, pound for pound, would produce an amazing amount of a very good composite between the two.
Lavos is doing this sort of thing with the rest of the planet's life, 'gifting' choice creatures with its power to create something wondrous, Of course, the result is something that isn't as natural as the rest of the animals nor as powerful as Lavos, but it's a creature that, eventually, 'ripen' into something that it would be able to make the most use of.
Zeal is an example of it going too well, however. Say you mix tobacco with roses, to get the insect-repelling qualities of the tobacco and the beautiful flowers of the rose. You indeed get just that, but it begins to overpower the rest of your garden and shrubs, smothering the other plants and stealing the rest of the nutrients out of the soil. The destruction of Zeal would be like Lavos uprooting these plants and using some weedkiller to get rid of the plant that's threatening the rest of its garden; Zeal was not only sapping from Lavos, which could be considered something akin to the plant happening to be poisonous, and that they're also repressing and slowly killing off the rest of the humans, which could be considered the rest of its 'garden.' And so, on those grounds, it expends just a bit more energy to rid itself of the metaphorical 'weed' of Zeal, and salvage what it can of the remaining creatures on the planet.