Krokodil is not an ester of morphine! It's not anb ester at all, but it is related to morphine. It's basically a slightly "simpler" version of morphine that turns out to be much more potent.
Desomorphine itself is much more potent than heroin or morphine by weight, and is essentially "just another opioid", although a potent one... compare levorphanol.
Krokodil is not pure desomorphine either! It's produced in a very unsafe and messy procedure from codeine, iodine, & various other reagents... The end result is a mixture that contains iodo-codeine (krokodile intermediate pictured), hydro-iodic acid (HI, hydrogenm iodide.. very corrosive), and occasionally other acids, tars, etc.
The reason these addicts are getting such massive tissue damage probably results from the injection of unpurified or poorly purified (highly acidic, corrosive) reaction mixture. From what I have heard a lot of them don't really care and only want to get their fix... quite sad.