I have been killing crimson mobs in the crimson for a while, now, and I still have not had a key mold drop.
Is there an easier way to farm key molds?
I want them so that I can get the vampire knives.
Unfortunately, all I can tell you is that Random Numbers are Random.
1 in 2500 is... not a whole lot, I'm afraid. And even after killing 2500 mobs, you're not guaranteed one.
So, if there's no way to get better odds, how can we maximize those odds? Simple -- spawn as many mobs as possible, then kill them. The key here, (ha!) is that it doesn't matter what mobs you kill -- Goblin Armies. Celestial Event Mobs. Pirates. Slimes. All are equally valid.
So, things that will help: