Factual error: In the opening scene for the episode, a character commits suicide by electrocuting themselves with a defibrillator. The device used was an AED (automated external defibrillator) which analyses the electrical rhythm of the heart and only delivers a shock when the heart rhythm is ventricular tachycardia or ventricular fibrillation. In this case, the device would not have delivered a shock as the person did not have both ECG electrodes attached and therefore would not have detected a cardiac rhythm. The person would also have had a normal heart rhythm.
Other mistake: There is no P-trap beneath the red sink in the Madrigal offices.
Chosen answer: There are several possibilities. Gus primarily does it to make an example out of Victor. Victor is one of Gus' most loyal henchman, and yet he had no problem killing him. Meaning it doesn't matter how loyal or valuable Walt is, he can still be killed and it won't bother Gus. However, Walt tells Jesse that perhaps Victor "flew too close to the sun." Meaning, Victor over stepped his bounds by learning how to cook meth. Gus didn't appreciate Victor's initiative and Gus only wants his employees to do the job they're given. Again, telling Walt and Jesse to do their job or else.
Bishop73
I think it was a combination of anger over Gayle's death and Victor being seen by witnesses makes Victor very vulnerable and therefore a dangerous liability to Fringe's empire.
By this point, the mugshot of Victor will have been spread around as a suspect in Gail's death. Victor had been come too hot to handle, so to say, so had to go.
Ssiscool