There is absolutely no need to cut off the horn. Cutting off something like a horn, or damaging a hornworm (or silkworm) before feeding can lead to an a mess- especially when the caterpillars are larger in size. The reason for this is that the inside of a hornworm is pretty much entirely liquid and under pressure. You cut a small hole in a hornworm or silkworm and it will all come gushing out. I've had a mantis take a bite out of a silkworm and half of the silkworm's insides came oozing out the hole almost instantly. Not pretty. Your chameleon is a highly specialized predator. It can handle an appropriately sized hornworm without you cutting off the horn.