I recently imported all my photos from my iPhone to my Mac. Now they are on both devices. I would like to delete some of them from my phone camera roll, but i want them to stay on my Mac. Will the photos/videos delete from both devices
Yes, in short, you are safe.
Depending on what software you are using, the only thing that could potentially happen is you delete your photo albums that you have synced to your iPhone, which is a manual process or you are syncing photos to your iPhone from your mac. However, since you stated you imported them, I don't foresee any issues.
NOTE if you are referencing iCloud, then also, you have nothing to worry about. iCloud stores all the files in your iCloud but if you delete them from different devices it only deletes it from that device, unless you haven't had time to sync it to your Mac, but again, strictly importing, meaning you dragging and dropping or clicking the import to iPhoto/Aperture etc button, than you are good to go.