While it’s true that toxapex can utilize poison that potent, it’s important to note that all poison-types have the ability to generate extremely dangerous toxins or harbor deadly microorganisms. For example, the toxic fluids that compose grimer and muk’s bodies are so potent that they can render an area virtually inhospitable to plants and any other wildlife for up to three years just by passing through it. The slightest scratch from a toxicroak’s claws, for another example, can outright kill an adult man in seconds. Dragalge’s poison is so acidic it can eat through three feet of solid steel, and koffing and weezing’s toxic gases are so volatile that they can ignite in a fiery explosion upon contact with open air. Poison-types by nature are extremely dangerous to handle, which is why they are one of the more difficult types to master.
However, the main thing to remember about them is that although they can kill, they won’t unless you pose a very real threat to them. Even grimer and muk, whose abilities you’d think would be passive, actually only secrete the fluids they’re known for as a last resort. During any other time outside of those dire circumstances, even a human can touch them without suffering any ill consequences whatsoever.
In other words, poison-types are fully capable of controlling how much poison they use at any given moment, and they understand that landing a lethal strike may do more harm than good when it comes to their own personal growth. That is, if they’re tame, and they kill their opponent, they know that means it may be a long while before they can battle again. If you encounter one in the wild, they know that if they kill you or your pokémon, then their chances of being caught and trained will decrease exponentially. They know they need you to either train them or to spar against them, so, in other words, they need you alive.
Of course, if you wander deep into a poison-type’s territory, then you may actually be in real danger, but with toxapex, that would mean diving into the ocean and intentionally touching one. While toxapex do indeed learn Venoshock (a distance attack) early on, they much prefer not attacking unless you come too close for comfort.
In short, anonymous, yes, toxapex can kill small pokémon or a human, but that doesn’t necessarily mean it’s dangerous to handle. So long as you make it clear to your toxapex that every battle is meant to be non-lethal, it will very likely understand and hold itself back. The only time you’ll need to worry is if your toxapex is scared for your life or its own life, but part of your job as its trainer is ensuring it always feels safe and comfortable, even on the battlefield.