Why did they inject Arthur exactly on the toes?

They are attempting to make Arthur's death look like an accidental drug overdose, so they are injecting him in a spot that drug addicts often use.

People who are addicted to IV drugs have to inject themselves a whole lot, and they are rarely careful how they do it. Over time, the injection sites develop scars, blisters, infections, etc. They're called track marks, and they are one of the most clear and obvious signs of a long-term addict.

Because having track marks carries a major social stigma, experienced drug users look for ways to hide them. One trick they use is to put the injections into places that are normally hidden from view, but still have thin enough skin that they can reach a vein. A favorite spot is between the fingers and toes: you have 16 of these spots on your body, so you can rotate through them, and people very rarely look in those spots. The toes, in particular, are one of the places on the body where it's not unusual to keep them covered all the time (as opposed to, e.g. wearing long-sleeved shirts in hot weather).

Experienced coroners, in turn, will frequently check for obvious injection sites between the fingers and toes when they see a suspicious death and suspect a drug overdose. By injecting the needle between his toes, the two assailants are hoping the ME will mistake Arthur's murder for a drug overdose and alleviate any suspicion on the man who had him killed.

They hid the tiny injection site between his toes. They put his sock and shoe back on. Left pill bottles out. The drug mimicked a heart attack. No one is going to find a pin prick between Arthur's toes. Nothing to do with drug addiction tho the killers knew a vein could be found there. "You better hit it."