I understand it's anecdata, but I still do not completely get how you can die from slow heart rate? Why heart shouldn't get a signal to beat more frequently if

I understand it's anecdata, but I still do not completely get how you can die from slow heart rate? Why heart shouldn't get a signal to beat more frequently if body starts to lack oxygen during sleep? Is it just because the blood is so thick and moving so slow it will clog and may produce a sudden heart attack? Thus you need to "manually" keep up the heart rate?