I mean:
- Literally fights Kid and tears his arm off (or someone in his crew did)
- Lucky Roo shot someone in the head in chapter 1
- Openly discusses matters with genocidal power-hungry, purveyors of "peace"
- Displays an extreme amount of competitive presence anytime Luffys name is mentioned or stated. Luffy has stated that he wants to physically beat Shanks
- Is framed after a Yakuza boss. Since Chapter 1, has been called "Kashira", which is Boss, not "Taisho", or Captain. Oda even went out of his way to make sure his intro box says "Pirate Boss" in Japanese. Pretty sure early ViZ (not current) fucked this up.
- His bounty references Oicho-Kabu, a famous game played by Yakuza
- Makes extreme bets he can't afford to lose. Instead of a pinky, he bets an arm.
There is a definite reason people feel that Shanks isnt exactly "good" guys. It's not a jump to a random conclusion, it's just like many other theories in this story that are based on hints Oda has layed down in the past. There is more reason to believe he's not exactly who he seems than not at this point.
He is by far the biggest enigma in this series for a reason. If he was 100% a good person, Oda wouldn't have made him so enigmatic. He wouldn't have made him meet the Gorosei so you, the reader, could question his motives without a reason.