Flowers already attracted insects. The evolution of flower into carnivorous flower is a smaller leap than a tuba or leaf into carnivory as they would also have to evolve to attract the prey.
Well there's a fundamental difference between a carnivorous plant and a murderous plant who just kills.
There are many plants who kill large number of animals all the time, as defense measures for example. But a carnivorous plant specifically kills the prey in order extract nutrients from it and use it to benefit itself, and it does so using specialized adaptations specific for that purpose and not just accidentally (like a broken tree branch falling down killing somebody down below doesn't make the tree carnivorous)
So a carnivorous plant needs to have ALL of these traits:
capturing or trapping prey in specialized, usually attractive, traps;
killing the captured prey;
digesting the prey;
absorption of metabolites (nutrients) from the killed and digested prey;
use of these metabolites for plant growth and development.
...in order to be considered a carnivorous plant.
Source: Carnivorous Plants: Physiology, Ecology, and Evolution from Oxford University Press
(HIGHLY recommend if you're interested in this topic, it's an extremely good book and the best comprehensive overview on carnivorous plants at the moment, with fairly up to date information from this rapidly developing field of study!
While all of these answers are mostly true, you have to go back in time. Darwin called it the abomniable mystery. Flowering plants and insects co-evolved rapidly roughly 150 MYA. So prior to flowering plants, there were few plants and insects and they were mostly generalists. The rapid expansion and explosion of insect diversity is deeply entangled with the explosion of diversity in angiosperms.
the oldest pollinators, prior to bees,butterflies and other insects. were beetles, as evidence of magnolias one of the oldest lineage of flowers, use only beetles.
Since as long as carnivore has been a word, probably. Carnivory is the noun for the act of eating meat, carnivore is the noun for a creature that eats meat and carnivorous is the adjective to describe a creature that eats meat.
Because they live in environments lacking in the nutrients that can be gained from invertebrates (e.g. in highly acidic soil). This allows them to compete better against other plants. I guess non-flowering plants don't need the same nutrients so can go without. Only a beginnner+ at ecological botany so someone here can surely explain better knowing lemmy!
I remember watching this farmer make a case otherwise, that ordinary bramble (?) is specialized to ensnare and trap fluffy sheep, providing chemical nutrients to the bush.
An interesting theory, but there are good reasons to doubt the claim, including the fact that woolly sheep are a recent product of human breeding, and that wild sheep are not even native to the same areas blackberries grow.
There's tonnes of blackthorn and a lot of sheep in the UK and I've never heard it to be problematic. Sheep ate pretty dim, but bramble is definitely not thorny/spiney enough to get caught bar the odd occasion. I'm sure I heard about a shrub (African maybe) that sheep can get completely ensnared in and die, but can't find it!