Fernando Tatis Jr. did exactly that Monday night ... with the bizarre scene making baseball purists go CRAZY over the 21-year-old!!

Here's the deal ... the San Diego Padres were up 10-3 on the Texas Rangers in the 8th inning when Tatis Jr. was ahead 3-0 in a count.

Padres manager Jayce Tingler says he instructed Tatis Jr. to take the next pitch and to NOT run up the score ... but the shortstop says he missed the sign from coach, and blasted the baseball to the moon!!

The homer was SICK -- it traveled over 400 feet and was Tatis Jr.'s 2nd of the night ... giving him 7 RBIs in total -- but EVERYONE was seemingly pissed at the Padre for it.

Check out the footage from the field ... Rangers manager Chris Woodward was furious -- and when Tatis Jr. got back to his own bench, you can tell his teammates weren't pleased either.

Afterward, Tatis Jr. says he was sorry for smacking the home run ... saying next time he'll likely take the pitch instead of crushing it and piling on runs.

"Those experiences, you got to learn from it," Tatis Jr. said. "Probably next time, I take a pitch now that I learned from it.”

Added Tatis Jr.'s manager, "He’s young, a free spirit and focused and all these things, and that’s the last thing we’ll ever take away. But that’s a learning opportunity, that’s it and he’ll grow from it."

In fact, the Rangers were so pissed over Tatis Jr.'s swing ... they nearly hit San Diego's next batter, Manny Machado, with a fastball in retaliation.

Of course, other baseballers rushed to Tatis Jr.'s defense ... saying the unwritten MLB rule needs to be abolished -- and the dude should keep swinging no matter what.

"Hey [Fernando] listen up," Reds star pitcher Trevor Bauer said of the controversy. "1) Keep swinging 3-0 if you want to, no matter what the game situation is 2) Keep hitting homers, no matter what the situation is"