zassou_x: This guide provide a smart, macro-based auto-attack solution for Diablo, no mod is required.
The guide is for Razer mouse user only, and best suited for mice with side or other programmable buttons.

Software required: Razer Synapse. (Old version shown, still valid for new version)

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1. Open Razer Synapse, go to macros tab, create a new macro and name it left click.

2. Click your left button in the blank area, and click stop. (You can edit the delay between button down and up)

3. Repeat step 1 and 2 but for the right click.
pic: telegra.ph/file/ed5371f6512ad93b108f7.png

4. Go to the mouse tab, click + button to create a new profile, name it Diablo.

5. Select Link Program, and choose the executable you play,
...\GOG_Games\Diablo\diablo.exe for classic version with battle.net support
...\GOG_Games\Diablo\dx\diablo.exe for "modernised" version
(Razer Synapse will switch to this profile if the current active window is this executable.)

6. Select button 5 (default: mouse button 5 or browser foward). Select Macro in the Button Assignment drop down menu. Assign left click to it, and select "Play while assigned key is pressed" as Playback Option. Click Save.
pic: telegra.ph/file/e5d7fb71d4d08c478d293.png

7. Repeat step 6, but assign right click macro to button 4 (default: mouse button 4 or browser back).

8. Select profile to the default ("Profile") or the previous one you were using.
Caution: If you missed this step, the profile will stay on Diablo and won't switch to others, and side button will still act as auto-fire left and right buttons.

Addendum: If you're using 3-button mouse like Abyssus, it is possible to bind a macro to middle mouse button, but for one key only.