Sly and slay are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Sly (pronounced “sl-eye”) is an adjective. It means clever, cunning, or calculating. For example, in many folk tales, foxes have a sly nature.
Slay (pronounced “sl-eh”) is a verb. It means to kill, especially in a particularly heinous or brutal fashion, or killing many people versus one person. The past tense is slew, the past participle is slain.
The following story uses both words correctly:
Sylvia was a very sly detective but she disguised it well. She often came across casual and unassuming. People would tell her more than they should have. Her latest case involved an executive assistant, who was found slain in an alley. The accused was the man who accompanied her to a nearby club, but he didn’t seem to know much about her. Clearly they hadn’t been in a relationship.
After weeks of investigating and asking around, she realized who the perpetrator was. The man had physically killed the assistant, but he was a hired hand for someone else. It had to be the boss, a matchmaker to busy, affluent types. Either the two had been having an affair, and the assistant had threatened to expose the story to the media. Or the assistant knew too much about finances or other problems and threatened to expose those issues to a wider audience.