What are two events associated with the impact of a comet or an asteroid with Earth that would cause a major extinction of life?
High temperatures and low temperatures caused by the asteroid/comet.
Depending on the size of the asteroid/comet all life below it will literally melt. Due to the high speed of the object it will compress all air below it and much like in a bicycle pump the air inside will heat up. But, because of the much higher speed and much grander scale of the asteroid/comet hitting the Earth all life below it will die due to high temperatures.
After the comet/asteroid impacts the Earth a massive crater will form spewing millions of tons of dust and ash into the atmosphere. All the dust in the atmosphere will block out the sun leading to a decrease in world temperatures and sunlight reaching the Earth. This will kill off plant life leading to mass-extinction.