I enjoyed reading your story. It has some nice twists and turns. The ending is foreshadowed, probably just enough. You have some really good descriptions leaving the reader with a mental picture. I do have a few technical things that you might want to consider.
1. The second sentence in the second paragraph might be made to read easier, somehing like, 'Three days after his wife's sudden death, he still...', perhaps.
2. There are a few places where a comma might help achieve clarity. ('Several time,...' or 'In the crowded living room, ...')
3. 'He knew that nothing infuriated the family...' Whose family? His? His wife's? Later on you clean it up but here you might want to say Kathy's family. Small point...
4. I don't know but usually police don't introduce themselves as 'Mr.', usually it seems to me that they would say detective or officer so and so. Of course, I've been wrong before.
5. 'Sears suit', how would John know they are from Sears? Would it be better to say cheap looking or something else. Wouldn't want to offend Penneys!
6. Your transition from talking to the detectives to 'It seemed different at the diner table.' is jarring to the reader. It should be double spaced at least or with a break like *** between paragraphs. The same applies from 'He could feel it.' to 'The flashing red...'
7. The twist at the end is good but it left me wanting a little bit more. I think you can expand it to find Timmy's sister and wrap up John's fate. What if establish the pill in the coffee by Timmy like you have but the detective gets a cell phone call and tells Jill that her brother-in-law crashed his car into a Starbucks and died instantly.
8. One thing that has helped me immensely is a little program called TextAloud (if you have Dragon Naturally Speaking, it will do a similar reading). You copy your story into it and let it read it back to you. It's great for hearing things that just don't sound right.
Overall, I enjoyed it and encourage you to refine it. You have great idea here! Maybe even better, it likely will be salable! It's headed for the 4.5-5 range. I'll stop before my review gets longer than your story. LOL