Find out who deleted a record in Salesforce

Check the Recycle Bin

The 'Deleted By' column will indicate which User deleted the record. If the record doesn't exist, you may still view the record via API before Salesforce permanently deletes the record.
 

Use the Data Loader

If the record doesn't exist in Recycle Bin, you'll need to log into Data Loader.
 

1. In the Data Loader, click Export All.
2. Select the object and choose the target for export file. Click Next.
3. Click Select all fields.
4. Create filter 'IsDeleted' = True. Click Add Condition.
5. Click Finish.
6. Open the CSV file.
7. Column 'Last Modified By ID' is the User record ID that deleted the record.

For Professional Edition or below, you may be able to leverage Dataloader.io to query for records that are no longer in the Recycle Bin.
 

1. Log into http://dataloader.io.
2. Click New Task.
3. Select Export.
4. Select 'Object,' and then click Next.
5. Click on Select All under Fields. Set Filter 'Deleted' = True. 
6. Click + | Next.
7. Expand 'Advanced.'  Set 'Export All Records' = checked.
8. Click Save and Run.
9. Click on hyperlink (ie...# success) to download the CSV file.
10. Open the CSV file.
11. Column 'Last Modified By ID' is the user record ID that deleted record.

Note: Dataloader.io limits exports to 10,000 records per query in batch mode, so you may need to narrow down your search at step 4.

For Dataloader.io support, please see Support for Dataloader.io.