Joined my current company as a Frontend Engineer 2 years back. They recently got funded and started hiring with a higher salary range. Not to brag but I'm pretty good at my job with 6yr of experience but my current salary makes me a lower mid-level engineer now and I'm the only frontend guy here.

So, now they're asking me to take interviews of the applicants who are applying for the senior position.

Why do people have to be such an assholes to the employees, man?

Talk to the managers? Definitely weird. It could just be an honest mistake, someone forgot to update your salary range.

That is extermely disrespectful.
Do not discuss. Hand in your resignation, and when they ask for a reason, say - I would continue here, but can make so much more somewhere else.
They have two options then:
1. They really don't appriciate you. And want you to leave. You are better off out of there.
2. They realize their mistake, and fix it immediatly. Bump in title, and comp package. Also - demand options, backdated to your original hire date.

Talking to the bosses, will only cause them to drag you along and delay resolving this.

Find another offer that fits what you'd expect given your expertise.

Then, with an offer in hand, go and negotiate as @magicMirror said.