I let my best friend down.
I'm sorry to hear that. How did you let him down?
I snorted his cocaine he left behind in the kitchen.
While it is bad that you accidentally set yourself up for heartbreak, there is no reason to take drugs, especially those that you see just laying around. Not only is it dangerous for your health, but because you essentially stole someone's property, illegal or not, they are obviously going to be very upset with you. Next time think about if doing something will hurt yourself and those around you, and if it will, then don't do it.
I have to atone for my loss.
It's good to want to atone for your mistakes. However, there is still the issue of taking those drugs randomly that were laying out. You never  know what someone may have pun in these drugs. These days a lot of coke is spiked with Fentanyl, which is many times stronger than heroin and causing a big upsurge in overdose deaths.