The hard truth is that lockdown is increasing the risk of more deadly strains of SARS CoV2 developing and propagating and we've understood why for centuries…
So viruses basically mutate like we do but at an exponentially faster rate, with thousands of mutations occurring all the time. So fast we can witness the evolution of viruses in real time [1, 2].
As we know from Darwin there is only one form of evolution (natural selection). The one in which a species mutates and those less effective "strains" die out and the species that is most effective at reproducing and surviving proliferates; viruses are no exception [3, 4].
The most effective virus is one that doesn't kill its host, so that the host continues to interact with and come into close contact with other potential hosts that the virus gets to spread to. If a virus kills someone before it can spread that strain of virus dies out and the one that doesn't kill becomes the most common. This is what would occur if we had no lockdown and allowed healthy invulnerable people to mix and spread non deadly mutations of the virus.
The only way we can avoid Darwinian evolution occurring is to interrupt the process with something like lockdown. Now rather than evolution occurring as mentioned, the virus predominantly spreads in places like hospitals and care homes, the kind of virus in hospitals is most likely a severe or deadly virus, hence why people are in hospital with it, it normally wouldn't be able to spread but as everyone is conveniently crowded together in these places, against natural evolution, it survives.
This wouldn't matter if normal evolution was occurring outside of hospitals and care homes with healthy individuals spreading non-fatal strains, encouraging increasingly less deadly mutations. But of course healthy individuals aren't allowed to mix and thus normal Darwinian evolution is interrupted.
If more deadly strains develop, this is why.
Bottom line: we can't stop the virus mutating but we can influence which mutations prevail.
[I would however add one caveat, it appears the new strains of the virus most recently reported in the news appears to be increasing in transmissibility rather than becoming more deadly thus far, despite lockdown, thankfully (and despite what Boris Johnson and others would have you believe based on their occasional highly politically motivated outbursts of scaremongering). This is perfectly in line with what we would expect based on the evolutionary tendency of viruses previously discussed; viruses wish to spread not to kill (as a conceptual aid, consider, for example, the human virus currently infecting planet earth).]
“I am not apt to follow blindly the lead of other men” - Charles Darwin
4. Domingo E, Escarmís C, Sevilla N, Moya A, Elena SF, Quer J, Novella IS, Holland JJ (June 1996). "Basic concepts in RNA virus evolution". The FASEB Journal. 10 (8): 859–64. doi:10.1096/fasebj.10.8.8666162. PMID 8666162. S2CID 20865732.