Would Google use their influence to kill a competitor for the own benefit? Definitely. They have a pattern of going against their own rules, their users' interests, and even the law when they stand to gain. To say this situation is any different in the absence of hard evidence, would be naive. (Edit, OP provided the aforementioned evidence that Google did have a competitive reason to do this.)
They have a rich history of stomping on little guys as well as being a masterclass in the, "do what I say, not what I do" sort of hypocrisy that always seems to come from those with great power. In my eyes, they've long lost the benefit of the doubt when it comes to matters like this. Google holds a lot of utility for me and I use their products daily, but I hold no illusions as to their predatory nature.
1) Google has claimed non-existent partnerships with their competitors in direct marketing of a competing product. They were literally calling their competitor's customers on the phone, saying they were a partner of the competing company, and hard selling that company's customers to sign up for their product.[1]
2) Google has the honor of receiving the largest civil penalty the Federal Trade Commission has ever imposed for overriding their user's privacy settings in Apple's Safari browser.[2] For some perspective: this fine was 0.1 percent of Larry Page's net worth, or what Google takes in over the course of about 5 hours.
3) Google search makes a regular habit of scraping other web sites and displaying ads against the content, something explicitly banned by their ToS for Adwords users.[3]
4) Google search also regularly shows upwards of 75% ads on a page. For instance, a search for "Kansas City real estate" returns you nine ads and three organic search results when viewed on a 2560 x 1440 screen[4] (15" MacBook Pro). This is explicitly against their policy, "Publishers should avoid site layouts in which the ads push content below the fold. These layouts make it hard for users to distinguish between the content and ads."[5] This also violates their add limit per page[6] laid out in their ToS.
5) This might be my favorite. If you Google "Scraper Site" they will actually scrape the explanation of the term from Wikipedia and serve up for you[7].
This what I could put together in about 20 minutes. Doing anything more than cursory research on this subject will leave with a very bad taste in your mouth about Google as a steward of the world's information. From convenient banning stories like this to lying about the data they collect in the first place and then lying about how long they keep it.
[1]http://business-ethics.com/2012/08/14/10058-is-22-5-million-...
[2]http://imgur.com/RjiKmOy
[3]http://imgur.com/ehIAsOa
[4]https://support.google.com/adsense/answer/1346295?hl=en#Diff...
[5]https://support.google.com/adsense/answer/1346295?hl=en#Ad_l...
[6]http://www.searchenginejournal.com/google-misuses-mocality-d...
[7]http://searchengineland.com/google-scraper-tweet-185684