In the context of discussion about mergers and acquisitions, I believe "killing a competitor" almost exclusively means purchasing a company so that it's no longer a threat. In the course of doing so, you assimilate the acquiree's users, or at least try to. This is the meaning I interpreted from the comment you responded to.
The rarer definition, which is that a company actually shuts down the product and doesn't make an attempt to migrate its users, strikes me as something more typical for a small startup that has not yet reached real market validation, but which has very impressive R&D.
Autodesk acquired Softimage and then 3 years later shut it down to great user uproar. Autodesk them tried to migrate them to Maya.
http://www.cgchannel.com/2014/03/softimage-died-to-make-max-...
https://www.change.org/p/autodesk-save-softimage
http://web.archive.org/web/20140519212219/http://e-roja.com/...
Trello covers a market that was not really served by any Atlassian products (or rather, market segments that weren't well treated).
Trello has a huge active following, and a lot of users who would otherwise not know Atlassian products.
Getting any sort of proper cross-selling going on would be great for their ecosystem long-term. People might start at Trello, but go to Atlassian products as their team scales and need other coordination tools.
I don't think you spend $425 million to kill a "competitor", especially when that competitor has a lot of paying customers anyways.