There was no way that the target could escape their grasp even if he turned out to be excessively powerful. They could take him down with numbers if they had to.
Blackbeard was a pirate and was least concerned with his people's lives in the face of benefits. As long as the benefits outweighed the value of their lives, he was willing to spill their blood like water if he had to.
"Don't be too sure. That kid is a dark mage and a sufficiently powerful one at that. There are rumors that he is an S rank mage but I don't think that's the case. At most he is a high-level A rank mage, otherwise the man I sent to take care of the Windsor family would have died in his hands already. Still his tricks are numerous and you should be cautious while taking him down."