Verb + 掉 (-diào) is a very productive resultative complement meaning "completely / away". 灭 alone is "to extinguish"; 灭掉 specifies that the elimination is complete and final. Compare 吃掉 (eat up), 删掉 (delete), 忘掉 (forget completely).
register
Colloquial in everyday speech and especially common in gaming and sports contexts ("beat" or "smoke" an opponent). For formal writing prefer 消灭 or 歼灭.
