"I don't see how" doesn't constitute legal proof, though, does it. "I highly suspect" and "I can prove" are two different claims entirely. And the lawyers rightfully feel like a police officer,
ostensibly in the legal function of his job, can easily provide enough reasonable doubt of "design to effect death" to prevent a guilty verdict.
So charging him with first-degree murder would
almost certainly end in the guy walking.
Besides, the harsh sentencing for first-degree manslaughter also makes it less reasonable to try for the far more unrealistic charge.