If you lift lockdown, lots of people get infects. It takes a few days, maybe a week to show symptoms bad enough to go to a doctor for testing. This is the point in time when cases begin to go up. Imagine if you lifted the lockdown for a day and went back.
One to two weeks after that, people will get more seriously ill, requiring hospitalization. Initially this can be treated easy at a (presumably) low-load hospital with no issue, the death toll is low.
One more week and you have the majority of cases in the hospital, this is where most people will die. A month after you lifted the lockdown.
Now imagine not locking down after that one day. After 1 month, you're at the peak of Day 1 infections, the next day the peak of day 2 infections etc. It compounds fast and can overwhelm a hospital.
Once a hospital is unable to care for all patients properly, triage will be applied. People with good recovery chances will receive treatment, those who are risk groups will not.
That's when your death toll will explode very suddenly.