Generally speaking, turns in a match aren't 50:50 scenarios. There are typically more than two outcomes. For example, you both sides have 5 'mons left, and you have a healthy bisharp on the field v.s. your opponent's healthy 4-attacks starmie. The best play here will be to use Sucker Punch as it means you are guaranteed to not lose your Bisharp. This is not a 50:50 as you are simply making the safest play you can as opposed to whichever player who makes the wrong move being severely set back by it.
Take a similar situation (1 HP Bisharp v.s. healthy 4-attacks Starmie), but this time make it a 2v2 situation (you have Bisharp and 1 HP Landorus-I, your opponent has Starmie and 4 attacks Zard X). In this situation, it is a 50:50 because whoever makes the correct play wins the match. Here are the possible scenarios here:
- Bisharp uses Sucker Punch and Starmie stays in. Starmie faints, you can sacrifice your Bisharp to the Charizard (dealing Sucker Punch damage as you go down) and Landorus wins the match.
- Bisharp uses Pursuit and Starmie switches. Starmie faints, you can sacrifice your Bisharp to the Charizard (dealing Sucker Punch damage as you go down) and Landorus wins the match.
- Bisharp uses Sucker Punch and Starmie switches. Zard X KOs Bisharp (as you can't switch in Lando for obvious reasons), Landorus kills Zard X, Starmie wins the game with the Rapid Spin KO on Landorus ('cause 100% accurate).
- Bisharp uses Pursuit and Starmie stays in. Bisharp faints to Rapid Spin ('cause 100% accurate), Starmie wins the match with the rapid spin KO on Landorus.
So there are two outcomes: Bisharp's team wins or Starmie's team wins. This is a 50:50.
Aegislash can be compared rather well to the second scenario.