Murray did a few things wrong, but this trade wasn't one of them. The purpose for acquiring Kane, is EXACTLY what we are seeing right now, a sought after asset, that the Sabres could use to acquire for good assets in the near future. You have to talent to build your talent.
Well, the way I see it is, Murray fell in love with players that he scouted as a scout. The Evander Kane trade was not a good use of assets, and up until the past 80 games or so, Kane was a pretty big disappointment. I don't think that anybody values Kane the way some on here do. Team chemistry is a really important component of a deep playoff run, and, while guys like Paul Guastad, Andrew Ladd, Martin Hanzal are inferior players, they are all also viewed pretty as reliable team guys with good character. Evander Kane, whether it's valid or not, does not have that reputation. More goes into it than just, "I have a feeling," but it's generally around the point that I don't think the Sabres will get good value out of Kane because he isn't the type of player that is typically sought after at the deadline. There a good chance the Sabres are going to be pretty disappointed with the return.
Also, I'm not sure I get your logic. You go get Kane so that you can speed up the rebuild. The general strategy of which is Murray's greatest failing. I know some posters are fond of pointing at not getting Babcock as the moment the rebuild failed, but at some point it's worth suggesting that the players aren't that great either. All on their own, the players just aren't that good. It's pretty hard to argue that the Sabres are in a better position now than they would have been if they had simply not made the Kane trade.