Leyland Cypress grow so quickly overall that they should not need (and probably won't benefit from) fertilizer application. Is the plant off-color or showing other signs of stress? If so, it's likely an issue revolving around root health, overly-wet soil, or the few pests and diseases they can contract. If fertilizer is warranted, they are best used as a soil-applied, dry, granular-type formulation rather than a liquid sprayed on foliage. This is because absorption will be improved through roots rather than foliage, and the dry fertilizers are time-release so they don't exude their nutrients all at once (which can damage roots or be wasted as the excess runs-off in rain).

Ant nesting activity is a good natural aerator for lawns - something that improves root health by letting the roots access more oxygen and helping excess water drain faster. Ants consume other insects and arthropods, so they would not cause the death of a plant as they forage. (In fact, they can be good predators of many plant pests.) When carpenter ants nest in a tree, they are only using wood that's already dead and beginning to decay, so damage to the tree began from other causes.

The product you mention is a general-purpose insecticide that does not specifically target ants. Because it can harm many beneficial insects and is not very targeted for any one pest, we do not recommend its use. Instead, if you need to reduce the ant colonies, a bait specifically formulated for ants (and outdoor use, in this case) would be best.

The insecticide you used also should not have killed a tree. Instead, one or more other causes contributed to its death, though it's hard to speculate what those were without having more information.

Lawns that host many burrowing insects like ants are probably struggling, since vigorous, healthy turfgrass grows densely enough above and below-ground that it would tend to discourage burrowers from establishing. It's possible an adjustment in lawn care, in order to help thicken-up the grass, will discourage ants and other burrowers in the future. We can't say what conditions need improvement without more information, but there's a lot of good information on the pages linked in this section: https://extension.umd.edu/programs/environment-natural-resources/program-areas/home-and-garden-information-center/yard-and-landscape/lawns-home-garden

Miri