So fallen angels have free will which makes them playable characters nice, it is interesting that Jim equates freewill with evil in this example and lack of free-will with monstrousness in other examples this means that Angels are in-human monsters and fallen angels something more.
Depending on how one wishes to characterize them, yes: Angels can be terrible, terrible monsters, as completely inscrutable to humans as the Fae, if you are between them and the Will of God. "Legion" is an excellent portrayal of the bloody, Old Testament, wrath of God brutality that had historically been attributed to His servants.
Conversely, insofar as "evil" is traditionally defined as "against humans" - and Angels are commanded by God to serve His creation (humans) - then yes, "falling" could, by dint of exclusion, connote "evil", but as Silverblaze notes, I would prefer to characterize that they exist in a state that is susceptible to sin. I believe an Angel could conceivably define its rebellion in a way that does not automatically make them anti-human. Again, taking "Legion" as an example, one angel defies His will and Falls in order to serve the greater Good. But these concepts get tinkered with in any supernatural setting that includes God, Angels and Devils.
Back to the DresdenVerse: I'm not certain whether or not the setting/Jim has said outright that the 30 Fallen bound to the Blackened Denarii comprise ALL of the Fallen to date, or if there may be an unknown or unnumbered cadre of Fallen who have not aligned with the Knights of the Blackened Denarius. For the sake of an open setting, I'd buy that some of the Fallen didn't actually join the Denarians, aren't specifically aligned with "evil" - that is, until I'm presented with canonical evidence to the contrary. And even so, I might condone it as one of those compelling "exceptions" like Thomas.