Even if I had no evidence, my default assumption would be that they were good until confronted with significant evidence that they were evil. And between good and evil there's a nice-sized neutral middle area which it appears you have transited in one fell swoop based on your sole piece of evidence.
I'm not sure what other evidence of goodness you seek than customers' experiences with them. I have found their prices competitive - as another poster said - lower on some items and higher on others. They have treated me fairly. I don't believe that they are engaged in any immoral enterprises. I agree it would be nice if they cooperated with Octopart but to say that makes them evil is absurdly hyperbolic.
I can immediately think of some (non-evil) reasons for Digi-Key to prefer not to have Octopart include its pricing. It has invested in a web site which provides an excellent user experience and they would prefer not to make it easier for customers to use Digi-Key's web site as a reference manual for lower cost competitors (whether or not you believe this is inevitable). They would like to avoid doing business with customers who are concerned solely about price. They would like customers to purchase their higher-margin items based on past experience purchasing lower-margin items even though the higher-margin items might be cheaper elsewhere.