Stingray venom has insidious, but powerful, localized tissue necrosing properties in humans.
What this means is, it kills any tissue it touches. If any part of Steve's heart was penetrated, fatal cardiac arrest would have ensured.
Very true. Of course, a direct puncture to the actual heart wouldn't be necessary. That venoms pretty strong so even a wound right near the outer layer of the pericardium would likely cause immediate life-threatening problems like tamponade or failure and the associated killers like v-fib or PEA.