A death combo is a combo that will guarantee a K.O. (just like an instakill but more innocuous-seeming at first) when performed once.
Because the damage done is very high, among other reasons, these allow one to disregard the all best tactics in fighting games; for this reason, the majority of players shun these. The minority opinion is that the degree of difficulty is high, making these high-risk and requiring advanced technique to pull off, so some people feel that it’s a romantic gesture in flashy play.
However, if one does not come to an agreement with their opponent beforehand, it can be cause for a real fight, and while it is not as ridiculous as the infinite combo, it is often seen as the tool of chickens. Some game centers, for the sake of avoiding trouble, forbid these, along with infinite combos and truly cheap moves.
Characters who have death combos generally encroach on high-tier, but all kinds of other characters have them, too; since it is probably tough to perform, simply having one doesn’t make the character better.
Recently, death combos that utilize dizziness have become more widespread.