Warning: My responses are not for the weak of heart. This is because compared to animals, humans on the whole are paragons of virtue, kindness, and mercy. This includes Hitler. Evolution is fucked up.
"Why when all creatures great and small, crawling out of the primordial ooze, did some animals evolve to be herbivores/carnivores/omnivores?"
Because that is a stable system. See, most evolutionary paths are failures, and the lucky ones are the ones that manage to find a niche. So a random creature mutates, and it gets the ability to eat plants. It does very well. Then, one day, there is a creature that eats meat. There is a ton of things out there to eat, none of them prepared to defend itself from a predator because they haven't evolved yet. It has a field day, it gorges itself, and is able to have kids all over the place. Basically, evolution throws out a bunch of creatures, some eat meat, some plants, some both, some the byproduct of some creatures chemical reaction on a Tuesday. The Tuesday thing dies out of starvation, enough meat eaters survive that can be supported by the rest of the population, the plant eaters are as successful as there ability to eat and procreate allow.
"How did some evolve to have venom that can do all kinds of fucked up shit and others didn't?"
How can a bird eating tarantula have the ability to throw it's hairs off it's body to defend itself but a deers only form of defence is it has eyes on the side of it's head and can run pretty quick?"
Because evolution does not actually favor the strongest and most badass: It favors the thing that has a kid that has a kid that has a kid...and so on. Deer have a lot of kids, and are good at finding food. Deer don't need to have scud missiles because they fuck a lot. They don't need anything cool.
"If animals eat the weakest or an abnormal baby did these evolutions occur? Surely the mother would have seen the mutation and eaten it."
Evolution works on the level of the gene, not the creature. And creatures arn't trying to guide evolution, they just try to have as much sex as possible, and get as much genetic material out there as possible. A mother has a deformed, creepy baby, the mothers genes are still programmed to pass along as many of its genes as humanly possible. The fact that it doesn't improve the herd any doesn't matter. Hell, maybe the mutated baby will get lucky and find a poor, down on its luck mate desperate to procreate with anything, or maybe it will become a rapist. Either way, the mother gets more of its genes out there. Yes, that is a horrible, dirty, nasty, inefficient, downright evil thing to do. Welcome to nature. Of course, some times, a baby is a true failure, and the effort to raise the failure just detracts from the mothers ability to make other offspring more successful. In that case, the mother will most certainly kill and eat the baby, or at least abandon it somewhere. Hell, maybe it will survive long enough to kill or rape something, in which case the mother still wins. The mothers job is to just throw its genes out there. It's the ability of everything else to murder it that culls out the weaker genes
If I made a new animal, which had no defence or offense, then plonked it down in the animals version of hells kitchen (Aus) how would it evolve and adapt to the environment? If it gets eaten then it can't send a message to it's kids saying "evolve a way to stop being eaten. It sucks!", so how does it over many generations evolve the ability or a way to stop itself being food?
Evolution doesn't make new animals. It takes old animals, and after countless random, horrible, painful mutations done by chance over millions of years, certain changes favor them in different niches. Lets say there is a food shortage. Thousands of a population die. There are some slightly bigger creatures, and some slightly smaller creatures. Smaller creatures need less food, and are generally more successful. Bigger creatures can murder smaller creatures. The smaller creatures leave for a different place. There bigger babies need more food, and keep getting devoured by all its brothers and sisters in the nest because there hungry and more numerous, and the bigger one has more meat on it. Or they get killed by the mothers because they are to inconvenient. Bigger animals get driven out of that herd, the females refuse to mate with bigger animals because it makes feeding children harder. The animals get smaller, faster, and if they moved to a place with, say, lots of climbing trees, the best climbers get mates, the worse climbers get eaten by ground predators. Meanwhile, the bigger animals that separated start killing bigger animals. They discover a new Niche, and they are more successful when they are better at killing. Smaller ones get killed by aggressive siblings, or thrown out and eaten by the mothers. Assuming they don't get too successful, kill everything around them, and then die a slow and agonizing death of starvation, these bigger creatures become powerful predators. In a few million years, these 2 groups won't even be able to mate anymore, they are too far apart evolutionarily. Now, you have a small, tree dwelling, scavaging, quick creature, and a large, deadly, ground carnivore, both just 2 different niches that were explored by the same creature once there was some evolutionary pressure.
Evolution is a process by which everything tries to murder everything else, some things find a hiding spot to protect against being murdered that are very different, and most things just die. In reality there is no such thing as, "More highly evolved". If you are still fucking and killing, you are successful.