That goal was entirely on McDavid.
Lazy pass to Bouchard at the line, then lazily skates back where there wasn't much danger, but then takes nobody and leaves his guy free to make that easy pass with zero pressure for the tip in. Ouch, McCaptain. That's one bad thing about him, he does tend to mope and wilt when he gets down.
That said, this must be such an incredibly frustrating series for the Oilers to be playing.