When did Shanks get scarred by Blackbeard? I've seen Shanks has a scar on his eyes because of Teach, since Luffy was just a kid, meaning that Ace was not a pirate at that time. Does that mean that Teach had a bad nature at the beginning? Because I think it was stated that blackbeard was a nice person until he saw the darkness devil fruit.
Marshall D. Teech (Aka Blackbeard)  was always a twisted and very calculating person, right from the start. He just never really showed his true personality to the rest of his crewmates, until he found the fruit he was looking for; he said so himself during his battle with Ace.


During Shanks conversation with Whitebeard, Shanks told Whitebeard the real truth about Blackbeard, when he stated that: "Blackbeard was quietly waiting for a chance. He didn't take the title of commander to be renowned, but rather to conceal himself."

Since Shanks and Whitebeard were on good terms, Shanks would most likely have visited Whitebeard a couple times previously, and apparently, during one of those visits, Blackbeard was able to catch Shanks off guard (which is still surprising, considering how adept Shanks is at Haki) and injured him.