Means Over Ends
The ends justify the means - why is this wrong? Aside from the obvious that knowingly doing something wrong is still wrong...
A simple example: I choose to steal money so I can give it to the poor. The end (giving to the poor) is good, however the means (stealing) is wrong.
If we allow ourselves to fall in this trap we're essentially saying God cannot work in a failed situation. The failure is the fact that there are poor people and God has not remedied their suffering, therefore we must resolve this any way we can - even if it means breaking the rules.
My example is very simple and easy to work around. But other situations are not so simple, and in their complexity the temptation grows to justify wrong solutions. I don't want to explore this much further except to say our limited understanding of God's ability to fashion victory from defeat is the stumbling point.
He brought about our eternal salvation from death on a cross, and did this without breaking any rules or stretching truths. We should trust Him with our other lesser defeats.