Hundreds of millions of people migrating from areas that become uninhabitable from sea level rise and increased temperatures, trying to get into any country that's less affected. Europe just almost tore itself apart over few million war refugees, imagine 100 times more people, with nothing to lose and a valid argument that it's all our fault they don't have a home now.
Food pressure, as arable land is lost to the sea and temperatures[0], plus collapsing ecosystems as insects and plants with high vulnerability to temperatures die off. Food prices increase, risk of food shortage increases too, and so does social unrest.
As some land is lost, other land becomes available, "defrosted" by the changing climate. With reduced agricultural capability and migratory pressures, nations will want to compete for both that new land and resources available on it. This could lead to war.
I could elaborate, but let me instead tell you to look at the problem in reverse: think of how little it takes to cause "mass horrible deaths". Most people live in cities now, and most cities are fed "just-in-time", which means there's 3-5 days worth of food in them. Imagine what happens if you break that supply chain for more than a week. Imagine this happening to multiple cities in a country simultaneously.
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[0] - You can't e.g. think that a +4°C Poland can just switch from wheat to bananas. Bananas may like the new summers, but won't survive the winters.