>>723358802
The killer7 has better damage than the Broken butterfly as soon as it's available to buy, and it outpaces the broken butterfly throughout the game for significantly less money even factoring in the fact that you get the BB for free.
The broken butterfly only out preforms the k7 until the very last merchant making it a better choice for 1 fight.
After that you get to new game plus and have access to the handcannon immediately witch is better than both in terms of damage/dps.