I wouldn't say it was an evil impulse. Serves the dumbphux right for attacking you.
I just did the same - sharing a continent with Cathy while peaceniking it on Emperor. I currently play for diplowins so never start a war, (which is no problem on Monarch) but as the Pangea turned out as three continents, my usual strat of letting the AI's batter each other down for me was compromised, and my Russian neighbours needed slowing down. Normally they'd get into a fight with another belligerent AI, but on this map I'm the only one sharing their border.
Anyway, when her troops poured over my border I paid her GPT for tech, took several hundred GP for some iron (ROTLF) and let her kick off the same turn - screwing her rep even further and surely guarranteeing her a poor show of hands in the UN.
Slightly OT note on war tactics
The war's not over yet (I'm waiting at least for the 20-turn alliances to end), and it's the defence-dominant inf/cav/arty era. I'm using pillage teams of inf/cav stacks in enemy terrain, while defending my border with arty to red-line their inf stacks on flat ground, before finishing them with vet cav (losses ~ 1 cav per 4 inf) - with infantry defending and directing the enemy to the kill zones.
On turns with no attack wave, arty pounds and weakens their border cities, taking with cav (heavier losses) when/if
totally battered. I rebuilt on the site with rubber and oil
- and hope to get rid of the two whose borders nick the two saltpeter tiles. That and the pillaged 40% of their empire should weaken them enough - especially with no oil, for the endgame.
If so, they'd be up for a tank-stamping, but I shouldn't need any more of their stuff and I definitely don't need the WW and a slow government. I also don't want the economy dip of consolidating and defending new terrain, whilst rushing for the UN.
So like the thread says - gains are available if attacked; even if SUC-Dealing (Seen U Coming) gets deemed a 'Sploit' (which it isn't IMO).