Dead Island Minimum System Requirements
OS: Windows XP / Windows Vista / Windows 7
CPU: Intel Core2Duo 2.66 GHz
RAM: 1 GB Memory
GFX: Nvidia GeForce 8600GT or ATI 2600XT 512MB
DX: DirectX 9.0C
HDD: 2.0 GB free hard drive space
Other: mouse and keyboard
Dead Island Recommended System Requirements
OS: Windows 7
CPU: Intel Core 2 Duo E6700 2.66GHz / AMD Athlon II X2 240
RAM: 4 GB RAM
GFX: GeForce 9600 GT 512MB / Radeon HD 3870
These are the FULL requirements for runiing Dead Island, and even includes the fact that you need a keyboard and a mouse, so it would appear that an internet connection is not required, or if it is, has not been mentioned.
If they haven't included that you need an internet connection to run this game, and you do, then it has been missold and you are entitled to a full refund, and don't let Game give you any of the "well it's been opened" rubbish, if it didn't tell you until you loaded it up that you had to download it then you HAD to open to find out. You can't add extra conditions once you've accepted the ones they offered, so you can't be responsible for not knowing that you needed to download until you opened and installed the game.
If you DO decided you want to keep it and don't want to kill your dongle I have 100MB unlimited at my place so as long as it's just a one-off to get it I have no problem with you using it