The nature will be destroyed
Volcanoes affect people in many ways, some are good, some are not. Some of the bad ways are that houses, buildings, roads, and fields can get covered with ash. As long as you can get the ash off (especially if it is wet), your house may not collapse, but often the people leave because of the ash and are not around to continually clean off their roofs. If the ashfall is really heavy it can make it impossible to breathe.

Lava flows are almost always too slow to run over people.
Some harmful effects of volcanic eruption are:
+ Volcanic ash highly affects the ozone layer and creates air pollution.
+ The gases and dust from the eruption may reduce sunlight for days, months or years.
+ Properties can be destroyed or damaged and so are the natural resources we have.
+ Lahar may be formed.
+ Lives of some residents and animals near the volcano that'll erupt may be injured or killed during the eruption.