Would you really begrudge someone who left for an opportunity that is better for them?
You are mixing up loyalty with persons with loyalty to companies. The former is important the latter is foolhardy.
One time a startup hired me and a buddy as contractors (expensive contractors) to help with their push to release. They had hired a bunch of junior (it seemed) marketing/sales/bizdevs types around the same time too. Two weeks later just about everyone was fired except the two expensive contractors (me and my buddy). I found out because I could hear some of the outgoing employees crying at their desk. One day they are eagerly working overtime to prep for a trade show, next day they are packing their stuff.
Soon thereafter, my project was canceled and I was reassigned to other work for a few weeks while I looked for somewhere else to work.
It's a Great Day at Hall Kinion was awesome on that gig too. I asked my handler to find me a new position, instead she setup an ambush meeting with me and the CTO to discuss why I wanted to leave. I couldn't believe it.