Ryder, who was returning to the New Zealand side after a calf injury layoff, cruised to 48 from just 27 balls after hitting two sixes and five fours.
However he looked to lose his rhythm once he got to 49 and eventually brought up his half century before giving up a soft dismissal by attempting a scoop shot which lofted straight into the hands of short fine leg.
McMillan said while other New Zealand batsmen had chances to win the match, the defeat falls squarely Ryder's shoulders.
"Yes the others had opportunities to win the game in terms of Franklin, Southee, Bracewell, Nathan McCullum, will all be disappointed they didn't bring New Zealand home. But the reason that New Zealand were put in that position last night was because of Jesse Ryder's selfishness in trying to get to 50," McMillan added.
"He changed the way he was batting. As soon as he got to 49 he took nine balls trying to get a single. He actually took about six trying to get a single and then he hit the fielder and thought 'Well I better hit one out of the park because this just isn't working for me'. There was a couple of swings and misses and all of a sudden New Zealand are under pressure where they need seven off the last over."
"This defeat falls squarely on his shoulders and he has to wear it."
New Zealand captain Brendon McCullum last night admitted his side blew a chance to claim the 2-1 series victory.
"When you look at where we were in the game and where we ended up, I guess we faltered," McCullum said, not calling it a choke, but not actually denying it either.
"We came under pressure and didn't measure up. Use whatever words you want but we didn't get the job done. It's bitterly disappointing."
McCullum did not point the finger at a "distraught" Ryder for the tumble - others could have finished the task, he said.
- HERALD ONLINE - additional reporting by David Leggat