"Dress to kill" is an English idiom describing someone who wants to put on attire so attractive that it impresses everyone who sees this person. The literal meaning of the phrase, that people could dress so well that it would kill anyone who sees them, gives the idiom its power. In actuality, the idiom actually builds off the English slang meaning for "to kill," which informally means "to impress." If someone were to "dress to kill," he or she would be so stylish that everyone who sees the clothing would be overwhelmed by it and how well it is worn by the person in question.
Idioms are phrases that gain more meaning through popular usage than what they would otherwise have if taken literally. These idiomatic expressions allow for speakers to talk in a way that is familiar to folks who know the expressions. The idioms also add a little flavor to conversation since they often use exaggeration for humorous or dramatic effect. One such exaggerated idiom is the phrase "dress to kill."
When someone uses this particular phrase, it means that he or she is describing a person who is almost outlandishly stylish. Often, the phrase "to kill" is an exaggerated way of saying that a person's dress or appearance will have an exaggerated effect on observers. In that case, people who know how to "dress to kill" have the ability to wear clothes that leave everyone in their presence in awe. It requires that the clothes be stylish and not so garish that they're tasteless.
As an example of how this phrase might be used in a sentence, imagine someone is trying to describe the kind of attire that a person needs to wear on a job interview. He might say, "You better have a nice suit to wear tomorrow, because you'll need to dress to kill to impress those executives." The literal meaning of the phrase would suggest that the person is going to try to kill his would-be employers, but, obviously, that is not the case. By contrast, he simply wants to impress them.
The phrase "dress to kill" and its adjectival counterpart "dressed to kill" often have some connection with the world of espionage. Popular spy literature and movies often depict the heroes of those stories as being impeccably dressed as a way to throw potential enemies off-guard. These characters may be literally "dressed to kill," considering that they are often required to kill their foes.