My spies are either insane, or at risk of going insane.
Adopting a number of identities for an extended period of time can cause severe psychological trauma, unless they have a powerful anchor that can allow them to retain a "base" identity.
Like the best actors, the best spies BECOME their characters. They can't respond to their own name, because that ISN'T their name. Over time, conditioning themselves to do so can lead to an identity crisis, which could potentially be severe as DID (that's disassociative identity disorder, or multiple personality disorder in common parlance) in which they slip into different identities without meaning to because they are no longer able to discern fact from fiction.
Because I have a raging hard-on for psychological conundrums (I nearly double-majored in psych, I may finish up that psych degree eventually), I generally make a point of providing "good" operatives with something that anchors them to their "real" identity, whereas "bad" operatives are somehow deprived of that anchor, leading them to first go rogue, and eventually go insane.
Now, my current setting is actually the unholy child of the American Civil War, Victorian England, Imperial China, and Renaissance Italy, with a varied bit of mythology/folk tales and H.P Lovecraft thrown in for good measure. Spies tend to be associated with a given City-State or Nation-State, with the odd free agent who is only employed out of desperation.
It's a bit a cheesy, but a quote from SWTOR encapsulates a spy's tactics well: "Anonymity...deception...these are your tools..." In general, they prefer to avoid killing and the headaches that are associated with it, and good spies can generally find a way around it. It's much better to arrange a scandal to remove an inconvenient politician, as opposed to potentially make him/her a martyr with assassination. If the headache can't be avoided, however, then the best weapons are ones other people wield for you.
(con't...maybe)