Risk of starting an altcoin

[Was]

So I was thinking, if someone were to create yet another altcoin using SHA256 as the hashing algorithm, and the coins ended up being worth something before the hash power was high enough to secure the network from a 51% attack, what exactly would you be able to do with, say, 75% of the hashing power?

Choose which transactions go through, and do not. But what about the ability to double spend?

Thanks!

[bhokor]

well i think you can fuck the network for example you can choose to not allow a tx if it dont pay a abussive fee , or make a double spending

[Rias]

With 75% you can do 51% attack more efficiently I guess. You will be able to double spend the coins easier.
https://en.bitcoin.it/wiki/Double-spending

[MisO69]

You can modify the code in the wallet so that you get very low difficulty, then mine a whole lotta blocks in a few minutes, then all other nodes accept those blocks since you have the most hash on the network. So you can mine out the coin if u really wanted.

Well, that's how I think it works anyways so if I'm wrong then someone chime in and correct me.