No, she didn't murder him. Honestly, such conjecture over two fictional characters.

If we want to ask the
really important question, we must ponder this:
Wouldn't Mr. Calvert have been reeeeeeeally pissed off to find out that his wife of sixty plus years and mother of his children didn't come join him in the hereafter and stayed instead with this skinny kid she knew for three days back in 1912 ten years before she and he got married? You fucked him once, honey, and you're spending eternity with him?
Rose was independent. A newly independent suffragette lady of the 20th century. She didn't have to get married "back in the 20s" per whatshisname the funny fat guy at the beginning of the movie. No one was forcing her this time. Presumably she loved the guy.
So she goes to be with the skinny little nerd in the afterlife?

Or maybe since they're in heaven it's a Big Love type scenario, only Rose gets to be the husband? She and Mr. Calvert do the heavenly deed in the clouds Mondays through Thursdays and Jack gets her on Fridays and weekends?

Discuss.