Yes, viruses can kill people very quickly. Most viruses don't want to kill their host because it stops them spreading. However, some viruses do have a high mortality, and this is a new virus that is still bouncing around mutating. So it hasn't settled down and realised it needs to do less damage to the people and animals it infects.
Covid19 and all viruses put pressure on the heart and immune system. Covid19 also puts heaps of pressure on the lungs and can cause blood clots in people. These blood clots are potentially lethal because they can stop blood flowing to the heart, brain or lungs. When the blood clots form in the lungs, the patient doesn't get enough oxygen and they suffocate. If the blood clots reduce flow to the heart, the patient has a heart attack. If the blood clots stop blood going to the brain or get into the brain, they cause stroke.