He struggled with the decision. Killing and murder are certainly two different things in the Bible, but it's still a serious and grievous thing to think about, plan for, and then ultimately take another person's life.
Here's where it came down for him: it was going to be more unloving to do nothing than to do this potentially sinful thing (thank you, Eric Metaxas, for this insight from Bonhoeffer's writings). And that ruled the day for him. Although the plot failed and ultimately cost him his life, it was the right thing to do.
And to all in leadership who might be paralyzed by a particular decision: is it more loving to "do nothing" in hopes of getting it right or to go ahead and act on a thought-through plan? Pastors in particular can get paralyzed by the "get-it-right" mentality. Sometimes it's action that is needed most (assuming of course there has been clear-headed thinking and the path isn't sinful and all the other ethical caveats needed here).
But that's just me thinking thoughts...