Answer:
If Human Beings Killed All Animals upon The Planet One after another out of Hatred or Vengeance then :
The Apex Predators like Lions, Tigers, Leopards, Cheetahs, Caracals, Mountain Lions, Jaguars, Pumas, Bears etc would Disappear - These Carnivores are Fearsome Predators and The Humans will generally avoid any Encounters or Close Interactions with them, unless of course they are armed or behind a Cage generally. So if The Carnivores disappeared then Nobody will be Afraid, they can go to Any Forest, Jungle, Grassland or Protected Reserve and they can Cut down Trees for Timber and Mine or Quarry a Forested area without any fear.
If The Carnivores are gone, Then The Herbivores will become abundant and Overgrazing, Soil Erosion will occur.
If The Herbivores are also removed, then we will have Birds - If Birds are also gone, then No Dispersal of Seeds, Control of Insects & Pests will suffer .
If Insects are also gone then No Pollination, No Honey, The Reproductive Cycles of Plants, Shrubs & Trees will also come down.
If Aquatic Animals like Crocodiles, Sharks, Whales, Dolphins, are made to become Extinct by Human Intervention, then The Dead Carcasses of Humans & Animals that flow downstream will Rot and decay and Water Quality will get affected.
If The Fishes are gone - Then The Water Quality will take a Beating & The surfaces of Lakes, Ponds, etc will get covered over by Algae, Sea weeds, and it will not be able to Replenish or Sustain itself and The Water will become unusable after sometime.
Similarly in Deserts if Camels were to be Exterminated, then there won't be alternate methods of traveling by land on deserts & plains (Only Aerial Travel might be possible by an Aircraft / Glider / Helicopter).
If Snakes are Eliminated then Rats will Breed Uncontrollably and Damage The Remaining Crops & Harvest produce.
If Scavenging animals like Hyenas, Wolves, Foxes, Jackals, Vultures, Eagles, Owls, Kites, Crows etc will be removed then The Rotten Carcasses of Dead Human beings & Animals will Decay & Decompose and Diseases might spread due to The Flies & other Disease Carrying Vectors like Cockroaches & Mosquitoes etc.
If Lizards, Chameleons & Spiders are gone, then Insects will Multiply manifold & they will Destroy Standing Crops by Feeding or by Boring through the Stems & Barks of Plants & trees.
Most of The Forests & Natural Habitats in The World are being Protected to Save Rare & Exotic Varieties of Species of Wild Animals, Birds, Reptiles, Aquatic organisms, Amphibians and other types of animals. If All of them are Exterminated in One go, then The Human beings would become Lonely and many of them are Leading their Lives due to these Natural Habitats in The Name of Conservation & Protection for a Long term basis.
It is Due to The Forests, Agricultural Fields & Nature that we are Leading Our Lives in Big Metropolitan Cities - Without them we too would perish.
Everything is Inter linked in The Food Chain or Ecological Cycle and we are Inter Dependent for Survival - We cannot become Independent completely and Overthrow The Animals (Big, Small or Insignificant) from their Right to Co-exist Peacefully upon This Earth in their Short lives.
Some Human beings also do have Pet Animals like Mice, Cats, Dogs, Hamsters, Reptiles for De-stressing Sessions or as playmates for children.
The Farm Animals like Buffaloes, Cows, Cattle, Sheep, Goats, Poultry and other animals also help to Sustain Human Life in one way or another with Dairy products, Wool, Meat or fur.
In Laboratory Tests Mice or Guinea Pigs along with Rabbits are used to Test Medicines for Future Use to Human beings so these Animals too End up paying with their Lives Eventually for Saving The Human Beings during Researches, Dissections, Experiments and what not?
So The Ultimate Beneficiaries are The Human Beings who are in Reality Exploiting Nature, Animals, Birds, Reptiles, Amphibians, Aquatic Life and other forms of Life for their Selfish needs.
If it were Not for The Poor, Hapless, Docile & Innocent Animals - Then who would Offer The Human beings Milk, Ghee, Curds, Butter, Dairy Products, Animal Fur, Animal Meats, Hides, Leather, Gelatin, Wool, Camel, Elephant & Horse Rides, Protection & Loyalty like Dogs, Amusing Pets like Cats, Entertainment in Wild Safaris like Lions & Tigers etc., The Birds who Sing throughout The Day, The Insects to Pollinate, The Scavengers to keep the Grasslands, Forests & Jungles clean. Animals too have their Own Roles & Responsibilities in an Ecosystem and they Live by their Own rules.
Ultimately, We Humans would be Poorer in their absence I guess in My opinion.
Explanation:
