kill·er 
(kĭlər)
(kĭlər)n.
1. One that kills: a disease that was a killer of thousands; a killer of new ideas.
2. Slang Something that is extremely difficult to deal with or withstand: an exam that was a real killer.
adj.
1. Causing death or destruction: killer floods.
2. Slang
a. Having impressive or effective power or impact; formidable: had a killer smile; made killer profits.
b. Extremely useful or indispensible: a killer web application.