A bit sad Takamura had to be taken out in such an anticlimactic way, but I guess it makes sense in context.
I guess we're to assume Slur just incorporated Takamura into his personality.
For real, what a anticlamactic way to send Takamura of.
If I were to guess this happened because the writer wrote himself into a corner - It would be out of character for Takamura to just stop trying to kill every main character and the main characters have no power to stop Takamura at all, the power gap is way too big.
Also, Takamura does not seem to be the kind of guy to get caught off-guard like that, if by taking his personality Slur got just as good as Takamura himself I would expect to have a big fight between the two not just a one-hit kill.