[This does not happen. It is not part of the fear of spiders continuity, even if Venom is straight out of there. It takes place out of time (and out of the timeline), and would roughly be Venom of 2077, and Tracer of 2076.]
"I am not your evil twin!" said Venom, popping onto the tall-chaired table outside the chippie.
"Yes, you are!" said Tracer, popping down next to her. "You're an assassin. With Talon. Evil! Also, you're in love with her." She snagged one of Venom's chips, just as her own arrived. "The most dangerous assassin our world's ever known."
"Yes!" Venom beamed, stealing a chip back. "A sexy, sexy assassin, and she's all mine, and I'm telling you - you're missing out." She popped the chip into her mouth, and, through potato, said, "Seriously, you have no idea."
Tracer laughed. "You can't even imagine how much Emily and I are in love. It's impossible! You're just too evil."
"Fffft," said Venom. "Emily's cute, I won't lie. But come on, luv, Amélie is perfection."
"If perfection means an aggressively overstyled shitehawk who loves killing people, maybe."
"Maybe," winked Venom, "you don't know anything about her. But if you're really as in love with this Emily frump as you think you are, I guess I don't feel sorry for ya after all. She probably deserves you. Tomato sauce for your haddock?"
"Tomato sauce? On fish? You daft?"
"You're so wrong about Amélie, it seemed likely."
Tracer laughed. "Oh, did I go too far? I'm sor - no, wait, no, I don't apologise to people like you, you're an assassin, and you like it."
Venom, in violet, snorted. "C'mon, you were military too, weren't ya?"
"Sure! Just like you," said the tangerine-clad Tracer.
"Everything exactly the same, right up to the Slipstream explosion."
"Mine didn't explode! I just disappeared. Then it crashed, but I wasn't in it."
"Yeah, but in my case, a bunch of plane parts disappeared, and the rest exploded. But that doesn't matter - up 'till then, everything's the same, right? We were both out to save the world, and if that means killing people, we do it, right?"
"Yeah, but not for fun. It was war, not what you do."
Venom pointed a chip at Tracer's face. "I bet my body count's lower than yours."
"What?!" exclaimed Tracer, splashing pepper on her breaded fish. "Not likely! I've barely ever killed any..." And she stopped, mid-sentence.
Venom nodded. "...anyone real." Gotcha. "Anyone human, or acting human. That's it, isn't it. Haven't you ever thought about all those other Omnic lives?"
Tracer stood her ground. "I have! But that was different. Null Sector at King's Row, every other Omnic incursion, it was open combat. We defended ourselves, and we saved lives doing it."
"Sure, I get that." Venom continued, munching on a piece of her fish. "Truth is, I even agree. But right or wrong - and I'm not saying it's wrong - how many Omnic lives did you end that day on King's Row? More than we would've, I bet. With a clean kill, we shift the future with one death. It takes you, what, five? Ten? A dozen? Two dozen?"
"My kill tally was 442," Tracer beamed, proud in spite of herself.
Venom laughed. "Funny woman. No, seriously, mate, how many?"
"Y'think I can't count?" Tracer protested, amusedly. "Four. Hundred. Forty. Two. I got a medal!"
Venom blinked, then blanched, as she realised her doppelgänger was absolutely serious, and stared confusedly at the woman who had called her an "evil twin." She put down her fish, and straightened in her chair. Bleedin' hell, she thought, as the weight of that body count struck her. So many dead. In one day. "That's, that's not killing. That's... mass slaughter."
Tracer nodded, munching another bit of fish. "It was a rout, you mean. They were throwing themselves at us the whole time. I almost died anyway - Mercy had to bring me back, once. And we saved a lot of old London."
"Yes, but..." Venom said earnestly, "Widowmaker and me..." She leaned forward again. "You don't understand. We've killed well less than half that many people our entire careers. Omnic and human, combined."
Tracer stared at her opposite. So... few?
"We'd have taken out the command corps. The rest would've been a mop-up. Most likely, the lot of 'em would've surrendered - or just left." Venom pushed aside her chips, frowning. "How many days like that you had, Tracer?"
The Overwatch agent lowered her head, but kept her eyes up, disturbed by her counterpart's reaction. "...several?"
Venom exhaled deeply, as her thoughts raced. I've stopped at a chippie with Pol Pot, she thought. "I'm... I don't even know, mate. We shape history with assassination, not snuggles, but your..." she waved her hands around, not knowing how else to say it, "exterminations... You're proud of them. You slaughter people en masse. It's..." she swallowed, hard. "Grotesque."
Tracer, firmly. "It. was. war."
"I know. And it ain't my world, maybe I can't judge. If you're Tracer, the Manic Pixie Murder Machine, that's what you are." Venom slipped off her barstool, looking grim. "But if you really think what I do is worse than that, I..."
Tracer glared at her opposite, defensive and a little angry. "What're you sayin', mate?"
Venom snapped, "What I'm sayin', mate, is that I'm pretty sure I'm not the evil reflection of you - I'm pretty sure you're the evil reflection of me." And she teleported away.
"Fine!" shouted Tracer, to the air. "Just for that, I'm eatin' your chips!" She rolled her eyes and shook her head. Assassins, she thought. Always so bloody pretentious.