I saw a few people confused about Oyang Zan's master scene crashing with we have formerly seen, I do not think it contradicts anything at all.
The assassins were sent after an stanian, Zan's master protected this stanian which made Manjusaka and Rock dude attack him because he was getting in the way of their mission and was a "traitor".
We see him being attacked by Manjusaka and Rock guy tells Ouyang that he broke his master's bones but we don't see Seven attacking him which makes sense since Killer Seven said once he only goes after targets.
However, in Ouyan Zan's flashback, we clearly see Seven killing his master.
I think if you connect dots, the reason is simple, he was putting him out of his misery.
because we also have this scene
In the movie announcement we got the same scene but we see Killer Seven slowly lifting his sword and with sad/worried expression, hesistating in killing who he has in front of him, even tho we have seen he is an amazing assassin.
It's implied this guy inspired Seven to help other people which could explain why he felt bad about killing him, we know he was empathetic deep down, he wasn't a target he was just some guy protecting a foreigner.
The colors and the master's designs are different but its's been over three years and designs tend to change a lot in production.
So yeah, Seven just mercy killed someone who got bone broken and poisoned, Ouyang Zan just didn't know the full story.