Albert Barnes Commentary Matthew 12:29

"Or how can one enter into the house of the strong [man,] and spoil his goods, except he first bind the strong [man]? and then he will spoil his house." — Matthew 12:29 (ASV)

Or else. He takes a new illustration to refute the Pharisees, drawn from breaking into a house. A man could not break into the house of a strong man and take his property unless he had rendered that strong man helpless. If he had taken his goods, it would therefore be sufficient proof that he had bound the man. So I, He says, have taken this property—this possessed person—from the dominion of Satan. This is clear proof that I have subdued Satan himself, the strong one who had him in possession.

The word else here means or how. The sense is: How, or in what way, can one...?

Spoil his goods. The word "spoil" commonly means now to corrupt, injure, or destroy. Here it means to plunder, to take with violence, as it commonly does in the Bible. See Colossians 2:8, 15; Exodus 3:22.