In the book The Sign of the Beaver, what did the stranger say that made Matt suspect that he might be a criminal?

When Matt asks the stranger, who gives his name only as "Ben", if he is traveling to the river, Ben replies,

"Not likely.  I'm keeping as fur off from that river's I can, till things quiet down"...Tell the truth, I got away from that town just in time.  Warn't nothin' they could prove, but they sure had it in for me.  So I says, Ben, I says...looks like now's the time to get moving".

Although Ben, in his rambling way, does not give any specifics, it is clear that he is on the run.  His words "warn't nothin' they could prove" indicate that he might have done something which was against the law.  Whatever it was, he does not want to go near the river town, because if he does, he is afraid of what will happen to him.  Ben's cryptic assertion that the people in the town "sure had it in for me" indicates that most likely he committed a crime, and would be arrested or otherwise have to face the wrath of the townspeople if he were to be caught back there.

Ben is just altogether a shady character.  He has no qualms about making himself at home in Matt's cabin, greedily snatching most of the meal Matt so kindly serves, and straighforwardly demanding some tobacco afterwards.  His small, glittering eyes take in everything about the small abode, and he impertinently asks if Matt is alone, as if measuring the risks if he should decide to take advantage of the situation.  Ben's malicious nature is made clear when Matt awakens in the morning to discover that the ne'er do well has taken off with his most prized possession, his father's gun (Chapter 3).