Should you go to emergency room for severe tonsilitis in adults?

Severe= needs doctor: Throat pain can be from strep throat infection (antibiotics are needed), mono, a cold or influenza (antibiotics usually not used), or from some other cause such as allergies or irritants. Usually, a person sees a doctor if the sore throat is bothersome, recurrent, or doesn't go away quickly (mainly to see about antibiotics). The doctor can evaluate to figure out the problem and start treatment.

Hard to say: Severe is a qualitative descriptor that will very with the person involved. In the US we have urgent care clinics & other settings where you would not compete with gunshot wounds/car crashes/broken bones/etc. If you are having trouble functioning you should be seen, but the venue may be different bases on what is available in your location.