Well first off its not Haku, there are only so many long hair hair styles that work with masks like that, they are bound to repeat at some point, second theres only so many designs you can make for a mask before it gets repetitive, for all we know in the mist village the designs might be a sign of ancestry or rank. Haku could have killed just about any mist Anbu person. that person might just so happen to have some connection to the one killed and thusly had a mask like the one Haku took.
Filler or no, if it was Haku they would have hinted somehow, like Naruto would have fliped out and tackled him to find out, i mean someone like Naruto wouldnt just sit there and think when he sees someone who got murdered right infront of him while on his first high rank mission.
That charicter must have been put in plase like that for the soul reason of making naruto have a flash back, and hopefully nothing further.
Hopefully