You know, while the Doctor on the television has been both merciless and violent, I'm not sure I can quite picture him ever declaring that there was no time for mercy.
I hadn't really thought of these stories as particularly brutal before (though I think there is so much out-of-character stuff in these 1960s comic strips that it hadn't occurred to me to consider that some were out-of-character in ways specific to the writer).
I've got a Telos book on the TV Comics/Countdown strips, and they speculate that there might have been some complaints from the BBC about the violence of some of the Second Doctor stories. In particular one where the Doctor invents a death ray and tests it on a giant (probably non-sentient) alien spider just because he thinks it's ugly, which is not something that should be believable for any version of the Doctor.
Oh yes! That's a step beyond this! Because I can sort of imagine the Doctor doing this and. doing it because of lack of time, but I find it hard to imagine him articulating that as a reason (even to himself).
I believe there's another second Doctor strip of that time where he shoots a giant spider with a ray gun while yelling "Die, hideous creature, die!" So much for "I used to have so much mercy."
I agree that I'm not sure the Doctor has ever been particularly merciful - though I think he very rarely explicitly acknowledges as such. However parrot_knight has suggested and londonkds (DW) have both suggested that the second Doctor comic strips were particularly out of character in this regard.
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