Which is a disingenuous way of framing it, essentially supposing that every single on of Arthas' decisions were badly motivated.
First, we have Stratholme. When people say "Arthas [was] the type of person who would murder a city", they always fail to understand that the death of the city was a foregone conclusion. This isn't just a matter of the infected grain, as we also know that Mal'Ganis was actively converting the citizens into undead. At the time there was no good option, only bad and worse ones, with Uther and Jaina taking (in retrospect) the most cruel action possible. Inaction, which Uther and Jaina chose, would have led to the entirety of Stratholme being raised into undeath, which we now know they would have been conscious for and is one of the reasons the Forsaken are so inhuman. Every person wouldn't have just died and been another member of the Scourge to march against Lordaeron, but would have been aware of their actions the entire time. The decision was either (1) purge the city and save people from undeath or (2) let the city be purged and raised into undeath. There was no third, righteous option.
Next, betraying the mercenaries and burning the ships is where Arthas begins to tilt away from justifiable; however, these actions were in direct response to Uther's. From Arthas' perspective, Mal'Ganis and whomever was controlling the Scourge in Northrend needed to be dealt with at all costs, and whatever price he or his forces needed to pay were insignificant in comparison. In this instance, it was either stop the campaign in Northrend immediately and allow the Scourge to grow in power or do something drastic to push the campaign forward. Muradin was right to be disturbed by Arthas' action, but this wasn't some evil action for the sake of being evil, but Arthas' viewed this as being an opportunity to finish the conflict and save Lordaeron.
Lastly, in his taking up Frostmourne, this is entirely irrelevant. Arthas reasons for taking up Frostmourne were in an attempt to find a way to kill Mal'Ganis, stop the Scourge threat, and save his people (i.e.: it was meant to be an act of self sacrifice). His actions during this time were also consistent with having good intentions, as he even attempted to heal Muradin with the Light before claiming the blade, only leaving Muradin behind when it wasn't effective. Regardless, the idea of him taking up a cursed weapon in an attempt to defeat an evil force somehow makes Arthas evil is silly.